How the Mitsubishi Error Code System Is Structured and Why It Matters
How the two-part code structure works
Mitsubishi Electric uses an alphanumeric fault code system across its residential and commercial ranges. Every code consists of a letter prefix and a number. The letter identifies the system area affected. The number identifies the specific fault within that area. Because of this two-part structure, a technician familiar with the system can often identify the fault category before connecting any diagnostic equipment — simply by reading the code the homeowner has written down.
Four prefix letters cover the majority of residential fault codes. E codes identify communication faults, typically between the indoor and outdoor units or between system components. P codes identify sensor and pipe temperature protection conditions. U codes identify outdoor unit protection states, including pressure and temperature limits. L codes, furthermore, identify indoor unit faults covering fan motors, drain pumps, and other indoor components.
The most important step before any reset
Write down the exact code as shown before switching anything off or attempting any reset. The code disappears when power is cut and may not reappear immediately if the fault is intermittent. Therefore, a written record of the exact code — combined with a note of what the system was doing when it appeared — provides the technician with the most valuable possible pre-visit information.
Photograph the display with your phone if possible. A photograph takes a few seconds and provides unambiguous evidence of the exact code, including any secondary characters that a verbal description might miss.
When a single reset is appropriate
A single power cycle is appropriate when a code appears for the first time with no obvious physical cause and the system has been operating normally until that point. Switch off at the wall isolator for a full minute, restart, and observe whether the same code returns within the first few minutes of operation. A code that clears and does not return was likely a transient condition. However, a code that returns after a correctly performed power cycle is persistent and requires professional diagnosis rather than further reset attempts.
Some codes should not be reset without addressing their cause first. P4 drain overflow, U1 high pressure, and U3 discharge temperature codes indicate active conditions where restarting the system risks progressive damage to protected components. These codes are identified in the tables below.
Every Mitsubishi Fault Code With Cause and Action
Select the prefix tab matching your fault code. Each table provides the fault description, the system's protective response, and the correct action for the homeowner. The urgency badge indicates whether a reset attempt is appropriate or whether the system should remain off until professionally inspected.
| Code | What It Indicates | System Response | Urgency | Homeowner Action |
|---|---|---|---|---|
| E1 | Indoor unit PCB fault. The main control board has detected an internal operating error or hardware abnormality. | System stops or reduces function to prevent further PCB stress. | One reset, then call | Perform one power cycle at the wall isolator. If E1 returns after restart, the PCB requires professional inspection. Do not continue resetting through a returning E1 code. |
| E3 | High-pressure protection fault in the indoor unit circuit. Indoor unit refrigerant circuit pressure has exceeded the safe operating limit. | Compressor stops to prevent damage from sustained high pressure. | Do not reset, call today | Switch off at the wall isolator. Check that the return air filter is clean and the indoor unit grille is not blocked. Call for service with E3 noted. Do not restart the system before the cause is identified. |
| E5 | Overcurrent protection in the outdoor unit compressor circuit. Electrical current to the compressor has exceeded its rated operating limit. | Compressor switches off to prevent electrical damage to the motor windings. | Do not reset, call today | Switch off at the wall. An E5 that returns immediately after a power cycle indicates an active compressor electrical fault. Furthermore, continued resets risk converting a repairable overcurrent fault into compressor winding damage. |
| E6 | Communication fault between the indoor and outdoor units. The serial communication signal cannot be reliably established or maintained. | System stops operation. Both units cease functioning until communication is restored. | One reset, then call | Perform one power cycle. Allow a short period after restart for the communication link to re-establish before concluding it has failed. A returning E6 consequently requires professional diagnosis of the signal wiring between units or the PCB communication module on either unit. |
| E7 | Outdoor unit fan motor fault. The fan motor has not reached its commanded speed or has stopped during operation. | System shuts down to prevent outdoor unit overheating from insufficient fan cooling of the condenser coil. | One reset, then call | Check the outdoor unit fan visually. Confirm it is not obstructed by debris or vegetation. Perform one power cycle and confirm the fan begins spinning during restart. A returning E7 indicates fan motor failure requiring professional replacement. |
| E9 | Electronic expansion valve fault. The valve that controls refrigerant flow between circuit components has not responded correctly to control commands. | System shuts down to prevent refrigerant circuit damage from incorrect flow control. | Do not reset, call today | Switch off at the wall. E9 requires professional refrigerant circuit diagnosis. The expansion valve is a precision component whose fault diagnosis requires specialist equipment and a licensed refrigerant handler. |
| Code | What It Indicates | System Response | Urgency | Homeowner Action |
|---|---|---|---|---|
| P1 | Indoor unit room temperature sensor fault. The thermistor measuring room temperature has failed or is reading outside its acceptable range. | System may continue operating in a limited state or shut down depending on how far the sensor reading has deviated. | One reset, then call | Perform one power cycle. A returning P1 indicates sensor failure or a wiring fault in the sensor circuit. As a result, operating with a failed room sensor produces incorrect temperature control behaviour and warrants a professional visit. |
| P2 | Indoor unit heat exchanger temperature sensor fault. The thermistor monitoring the evaporator coil surface temperature has failed or is reading abnormally. | System reduces function or stops to prevent coil operation without temperature monitoring. | One reset, then call | Perform one power cycle. A returning P2 requires replacement of the heat exchanger thermistor by a technician. The coil temperature sensor is critical for freeze protection and defrost cycle management. |
| P4 | Drain overflow detection. The float switch in the condensate collection tray has activated, indicating the drain is not flowing and the tray is at or near overflow level. | System shuts down completely to prevent additional water accumulation in the tray and ceiling cavity. | Do not reset, call today | Switch off at the wall immediately. Do not restart the system. The drain is blocked and the tray is at overflow level. Restarting therefore adds more water to a failing drain system. Call for same-day service and protect the floor and ceiling below the indoor unit while waiting. |
| P5 | Drain pump fault. The condensate drain pump has failed, stopped responding to control commands, or is not achieving its required flow rate. | System stops to prevent drain overflow from a non-functioning pump. | Do not reset, call today | Switch off at the wall. A P5 on a system with a condensate pump means the drain cannot flow regardless of whether the pipe is clear. Pump replacement is typically a same-visit repair as drain pump assemblies for current models are stocked as common service components. |
| P6 | Freeze protection. The evaporator coil temperature has dropped below the freeze threshold, indicating ice formation is occurring or imminent on the coil surface. | System reduces compressor output or stops the cooling cycle to allow the coil to return to above-freezing temperature. | Clean filter first, then reset | Switch off the system and inspect the return air filter immediately. A blocked filter is the most common P6 cause. Clean the filter and allow the coil to defrost fully before restarting. If P6 returns with a clean filter in place, low refrigerant charge is consequently the likely cause and a professional pressure check is required. |
| P7 | Vane motor fault. The motorised horizontal vane has not reached its commanded position or has stopped responding to directional control commands. | System may continue operating with the vane in its current position or stop depending on severity of the fault. | One reset, then call | Check whether the vane is visually obstructed by any object. Perform one power cycle. A returning P7 requires vane motor replacement, which is a standard service component for current Mitsubishi residential models. |
| P8 | Outdoor unit heat exchanger temperature sensor fault. The thermistor monitoring the condenser or outdoor coil temperature has failed or deviated outside its operating range. | System stops to prevent outdoor unit operation without coil temperature monitoring. | One reset, then call | Perform one power cycle. The outdoor unit heat exchanger thermistor replacement requires outdoor unit access and is therefore a professional service task. A returning P8 should be addressed promptly as the outdoor coil temperature sensor plays a role in defrost cycle management during heating operation. |
| Code | What It Indicates | System Response | Urgency | Homeowner Action |
|---|---|---|---|---|
| U1 | High pressure protection. Refrigerant circuit pressure on the high-pressure side has exceeded the compressor's safe operating limit. | Compressor stops immediately to prevent pressure-related damage to the refrigerant circuit and compressor seals. | Do not reset, call today | Switch off at the wall isolator. Check that the outdoor unit has clear airflow on all sides with no obstructions. Do not restart the system until the pressure cause has been professionally diagnosed. Continuing to operate through U1 codes consequently risks converting a manageable repair into a compressor replacement. |
| U2 | Low voltage or power supply abnormality in the outdoor unit. The voltage reaching the outdoor unit PCB has dropped below the minimum required for safe operation. | System stops to prevent damage to electronic components from under-voltage operation. | Check supply, one reset | Check whether other appliances on the same circuit are also experiencing power issues. If other appliances are normal, perform one power cycle. A returning U2 requires inspection of the outdoor unit power supply wiring and the circuit protecting the outdoor unit. |
| U3 | Discharge temperature protection. The temperature of refrigerant leaving the compressor has exceeded the safe limit, indicating the compressor is overheating or operating under excessive load. | Compressor stops to prevent thermal damage to compressor internals from sustained high discharge temperature. | Do not reset, call today | Switch off at the wall. U3 represents an active risk of compressor thermal damage. Check outdoor unit clearances before calling. Providing the U3 code and the outdoor temperature at the time of occurrence helps the technician assess the likely cause before arriving. |
| U4 | Communication fault between indoor and outdoor units at the outdoor unit end. Similar to E6 but identified at the outdoor unit PCB rather than the indoor unit. | System stops operation. Both units cease functioning until the communication fault is resolved. | One reset, then call | Perform one power cycle and allow a brief period for communication re-establishment. A returning U4 requires professional diagnosis of the signal wiring or the outdoor unit PCB communication module. Additionally, note whether the code appeared after any recent electrical work near the unit. |
| U5 | Current transformer fault in the outdoor unit. The component measuring compressor current for protection purposes has failed or is producing abnormal readings. | System stops. Without accurate current measurement, compressor overcurrent protection cannot function correctly. | Do not reset, call today | Switch off at the wall. Operating without functioning current measurement removes a key compressor protection mechanism. Therefore, U5 requires professional outdoor unit PCB inspection and component replacement before any restart. |
| U6 | Compressor overload protection. The compressor motor has been detected operating outside its rated current or power limits for a sustained period. | Compressor stops to prevent winding damage from sustained overload conditions. | Do not reset, call today | Switch off at the wall. U6 indicates the compressor has been operating under stress. Check outdoor clearances before calling. A returning U6 after a single power cycle suggests a developing compressor condition that consequently requires professional refrigerant circuit assessment before the system returns to regular service. |
| U8 | Outdoor unit fan motor fault. The condenser fan motor has not reached commanded speed or has stopped during operation. | System shuts down to prevent condenser coil overheating from loss of cooling airflow across the coil surface. | Check clearances, one reset | Check that the outdoor unit fan guard is free of debris and vegetation. Perform one power cycle and confirm the outdoor fan begins spinning during restart. A returning U8 indicates outdoor fan motor failure requiring professional replacement. |
| Code | What It Indicates | System Response | Urgency | Homeowner Action |
|---|---|---|---|---|
| L3 | Indoor unit electrical box overheating. The temperature inside the indoor unit electrical compartment has exceeded the safe operating limit for electronic components. | System stops to prevent heat damage to PCB components and wiring insulation in the electrical enclosure. | Do not reset, call today | Switch off at the wall. L3 indicates an active thermal condition inside the electrical compartment. Do not restart the system. Additionally, check that the indoor unit grille is not covered or blocked. Call for professional inspection before any restart. |
| L5 | Outdoor unit DC overload protection. The DC power conversion circuit in the outdoor unit has detected an overload condition beyond its rated specification. | System stops to prevent damage to the DC power conversion components and the compressor drive circuit. | Do not reset, call today | Switch off at the wall. L5 represents an active electrical overload condition in the outdoor unit. A power cycle without addressing the cause risks immediate reactivation of the same overload condition. Therefore, professional outdoor unit diagnosis is required before restart. |
| L8 | Indoor unit fan motor speed abnormality. The indoor fan motor has not reached its commanded speed or is operating outside its normal speed range. | System stops or reduces function to prevent motor damage from sustained speed deviation. | One reset, then call | Perform one power cycle. Check that the indoor unit grille is fully open and the filter is clean, as restricted airflow can cause the fan to struggle against excessive resistance. A returning L8 indicates fan motor fault requiring professional assessment. Furthermore, L8 combined with an unusual noise from the indoor unit points to fan motor bearing wear requiring prompt attention. |
| L9 | Indoor unit PCB component fault. A specific component on the indoor unit main control board has failed or is producing readings outside its operating range. | System stops to prevent further PCB damage from the identified component fault. | One reset, then call | Perform one power cycle. A returning L9 requires professional indoor unit PCB inspection. Depending on the specific L9 variant and the model, this may involve component-level repair or PCB replacement. Parts availability should be confirmed before committing to repair on older systems. |
Four Steps to Take When a Fault Code First Appears
Following these four steps in sequence protects the system from additional damage and gives the technician the information needed to arrive prepared for a same-visit resolution in the majority of fault code cases.
Record the exact code immediately
Write down or photograph the exact alphanumeric code as shown on the display and remote before switching anything off. The code disappears when power is cut and may not reappear immediately after restart. Therefore, this single step provides the most valuable diagnostic information available before a technician visit.
Look up the code in the tables above
Find the code in the appropriate prefix table and read the urgency rating. Urgent codes require switching off immediately without a reset attempt. Service codes allow a single power cycle. Reading the code before taking any action consequently prevents the mistake of resetting through a code that requires immediate shutdown.
Perform one reset only if the table permits it
For codes rated as allowing one reset attempt, switch off at the wall isolator for a full minute and restart. Observe the system during the first few minutes of operation. If the same code returns, the fault is persistent. In that case, do not attempt a second reset — switch off and proceed to step four.
Call with the code and your observations ready
Call 03 4232 6971 with the exact fault code, the model number from the indoor unit label, whether the code cleared or returned after the reset, and any physical symptoms observed alongside the code. As a result, our Melbourne team can identify the most likely cause and bring the relevant components for a same-visit resolution.
Fault Codes Grouped by Response Urgency
Three response categories cover every fault code in the Mitsubishi residential range. Use this grouping for a quick urgency check before looking up the specific code in the tables above.
One Reset Attempt Is Appropriate
These codes may reflect transient conditions. One power cycle is reasonable. Call if the code returns.
Indoor PCB fault, one reset, then call if it returns
Communication fault, one reset, allow a brief period to re-establish, then call if it returns
Outdoor fan fault, check clearances first, one reset, then call if it returns
Room sensor fault, one reset, then call if it returns
Coil sensor fault, one reset, then call if it returns
Freeze protection, clean filter and allow full coil defrost first, then restart
Vane motor fault, check for obstruction, one reset, then call
Outdoor sensor fault, one reset, then call if it returns
Power supply issue, check supply, one reset, then call
Communication fault, one reset, allow a brief period to re-establish, then call
Outdoor fan fault, check clearances, one reset, then call
Fan motor speed fault, check filter and grille, one reset, then call
Indoor PCB component, one reset, then call if it returns
Switch Off, Address the Cause, Then Restart
These codes have a specific homeowner action required before any restart is appropriate.
Switch off, clean filter and allow full coil defrost, then restart. If P6 returns with a clean filter, call for a refrigerant pressure check.
Switch Off Now, Do Not Reset, Call Today
These codes indicate active conditions where restarting risks converting a manageable repair into a significantly more expensive one.
High pressure indoor circuit. Switch off, call today.
Compressor overcurrent. Switch off, call today.
Expansion valve fault. Switch off, call today.
Drain overflow. Switch off immediately, protect ceiling, call today.
Drain pump failure. Switch off, call today.
High pressure outdoor. Switch off, check clearances, call today.
Discharge temperature. Switch off, call today.
Current transformer fault. Switch off, call today.
Compressor overload. Switch off, call today.
Electrical box overheating. Switch off, call today.
DC overload. Switch off, call today.
Fault code identified and ready for a service visit?
Our Melbourne Mitsubishi specialists arrive prepared for your specific code in a single visit.
What Melbourne Homeowners Ask Most About Mitsubishi Error Codes
Direct answers to the fault code questions that come up most often, covering both the codes themselves and the decisions homeowners face when a code first appears.
The tables cover the most common residential codes
The tables above cover the most commonly reported fault codes across the current Mitsubishi Electric residential range. However, older models, commercial systems, and specific sub-variants within a model family sometimes produce codes that follow the same prefix structure but use numbering outside the ranges listed. Ducted systems and VRF commercial systems also use codes that differ from the residential split system range.
How to respond when the number is unfamiliar
If your code uses an E, P, U, or L prefix with a number not in the tables, the prefix still tells you the fault category. An unfamiliar E code is still a communication fault. An unfamiliar P code is still a sensor or protection condition. Use the prefix to determine whether the code involves compressor protection — which warrants a switch-off approach — or whether it is more likely a sensor or communication fault that allows a single reset attempt. Call us with the exact code and model number and we can identify the specific meaning before you commit to any action.
The active flag clears, but the history remains
When a fault code clears after a power cycle and the system returns to normal operation without the code reappearing, the active fault flag in the operating memory is gone. The system operates normally. However, the PCB fault history log in non-volatile memory retains a record of every fault code that has been generated, including codes that cleared after a single power cycle.
Why that history matters during a service visit
This stored fault history is accessible through the service diagnostic menu that a technician connects to during a professional visit. As a result, the history provides valuable information about whether the current fault is isolated or part of a pattern of developing conditions. When you call with a fault code, mentioning any previous codes that have appeared and cleared — even if they seemed minor at the time — helps the technician interpret the current fault in context.
Yes — transient codes store silently
Transient fault codes that activate briefly and clear themselves are stored in the PCB fault history even if the homeowner never observed the active code on the display. These self-clearing codes often represent brief protection activations that the system resolved automatically. They are relevant context for a technician even when they produced no observable symptom at the time.
App-connected systems offer better visibility
Some Mitsubishi systems equipped with Wi-Fi adapters send push notifications to the homeowner's smartphone when fault codes activate. This consequently provides real-time visibility of fault events that might otherwise go unnoticed — particularly codes that activate overnight or during periods when the homeowner is not present. If your system has a Wi-Fi adapter and companion app, checking the app's fault notification history alongside the physical display provides a more complete picture of recent fault events.
The code itself does not void your warranty
A fault code that activates due to a component failure within the warranty period does not itself affect the warranty. The warranty covers component failures that are not attributable to inadequate maintenance or improper use. However, a P4 drain overflow code that results from a blocked condensate drain on a system without documented annual servicing may be assessed differently to a P4 on a system with a complete service record — because the drain blockage is a maintenance condition.
Why the service record is important for claims
The annual service record is the documentation that distinguishes a warranty-supported component failure from a maintenance-related fault. Our service reports are formatted to provide the documentation that supports a warranty assessment if a claim becomes necessary. As a result, if you have received a service report from us in the past year and a fault code activates indicating a component failure, the service record establishes that the system was correctly maintained up to the point of failure.
Multiple developing conditions, or one root cause
Different fault codes appearing across separate failure events can indicate two different things. The first possibility is multiple independent developing conditions, where different components are approaching failure at similar rates after years of operation. An older system without service history may therefore generate a P6 freeze protection code one week, then a U4 communication code the following week, as multiple marginal components begin reaching their failure threshold.
When different codes share the same underlying cause
The second possibility is that the codes are related to a single underlying condition that manifests through different protection circuits depending on operating conditions. A marginal refrigerant charge, for example, can produce P6 freeze protection codes during cooling, U3 discharge temperature codes during heating on cold days, and U1 high pressure codes during high ambient temperature operation — all on the same system. Consequently, a pattern of apparently unrelated codes on a system that has not been recently serviced warrants a full refrigerant pressure check as the starting diagnostic point.
Every Code Has a Cause and Every Cause Has the Right Response
Mitsubishi fault codes are precise communications about specific system conditions. Reading the code before taking any action, using the tables in this guide to identify the correct urgency level, and following the four-step response sequence consequently protects both the system and your home while providing the information that allows our Melbourne team to resolve the fault efficiently in a single visit.
Call 03 4232 6971 with the exact code, the model number, and the results of any checks you performed before calling. Our Mitsubishi specialists carry the most common fault-related components for residential models in every service vehicle. Same-day attendance is available for urgent codes. For related reading, see our Mitsubishi reset guide and our blinking lights guide.