Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Mitsubishi Mr. Slim Error Codes: Complete Guide
Mitsubishi Mr. Slim is the commercial line of Mitsubishi Electric ductless and ducted mini-split systems, covering PUHZ, PEA, PKAA, PCA, and similar series from 1 to 5 tons. Error codes are displayed on the wired remote controller (PAR-31, PAR-40, or similar) or via the outdoor unit LED diagnostic sequence.
Reading Mr. Slim Error Codes
- Wired remote: Fault code displays directly on screen
- Outdoor LED: Count flash patterns on the OPERATION and TIMER LEDs
- Wireless remote: Shows “CHECK” with a blink count — count and look up in this guide
Mitsubishi Mr. Slim Error Code Table
P-Codes — Protection/Safety
| Code | Description | Common Cause |
|---|---|---|
| P1 | Indoor unit intake temperature sensor fault | Check sensor at return air |
| P2 | Indoor unit pipe temperature sensor fault | Check liquid pipe sensor |
| P4 | Drain sensor fault | Float switch or drain sensor wiring |
| P5 | Drain pump fault | Condensate pump failure or blockage |
| P6 | Freeze protection trip | Low refrigerant, low airflow |
| P8 | Pipe temperature fault | TXV issue, refrigerant flow problem |
| P9 | Indoor fan motor fault | Fan motor or PCB |
| PA | Four-way valve fault | Stuck or failed reversing valve |
| PJ | Capacity mismatch | Indoor/outdoor capacity incompatibility |
E-Codes — Indoor Unit Electrical
| Code | Description | Common Cause |
|---|---|---|
| E0 | Remote control signal fault | Remote wiring or signal issue |
| E1 | Indoor unit PCB fault | Replace indoor control board |
| E3 | Indoor unit fan motor fault | Motor winding or capacitor |
| E4 | Indoor unit serial communication | Wiring between indoor boards |
| E5 | Indoor unit AC current protection | Overcurrent — check wiring |
| E6 | Compressor overcurrent | Check compressor and supply voltage |
| E7 | Fan motor overcurrent | Fan motor drawing excess amps |
| E9 | Electronic expansion valve fault | EEV coil or position sensor |
U-Codes — System/Communication
| Code | Description | Common Cause |
|---|---|---|
| U1 | Reverse phase detection | Check 3-phase power phasing |
| U2 | Power module fault | Outdoor IPM (inverter module) fault |
| U3 | Charge failure (first run) | Incomplete refrigerant charge |
| U4 | Serial communication fault | Check indoor/outdoor wiring |
| U5 | Abnormal indoor unit combination | Address conflict, unit mismatch |
| U6 | Compressor motor fault | Check inverter drive module |
| U7 | Inverter PCB fault | Outdoor inverter board failure |
| U8 | Outdoor fan motor fault | Fan motor or outdoor PCB |
Most Common Mr. Slim Faults
P6 — Freeze Protection Trip
Most frequent fault in cooling season:
- Check air filter — replace if dirty
- Check indoor blower motor and capacitor
- Check refrigerant charge — suction superheat should be 8–12°F
- Verify all supply and return registers are open
P4 / P5 — Drain Fault
- Check condensate drain line for blockage
- Test float switch operation — should open with water present
- Check condensate pump operation if pump-assisted drain
- Check drain pan for overflow
U4 — Serial Communication
The most common Mr. Slim multi-zone fault:
- Check M-NET or TB5 wiring connections at each indoor unit
- Verify outdoor unit address switches are correct
- Check for damaged shielded wire on long runs
- Verify power supply to indoor unit control boards
E6 — Compressor Overcurrent
- Check supply voltage at outdoor unit (should be within ±10% of rating)
- Check compressor insulation resistance (should be >1 MΩ to ground)
- Check inverter module output — replace IPM board if shorted
Mr. Slim Parts Reference
| Part | Notes |
|---|---|
| Indoor pipe temperature sensor | Part varies by model — 10K or 5K thermistor |
| EEV coil | Model-specific — match wiring harness |
| Indoor PCB | Match part number from unit nameplate |
| Outdoor inverter PCB | High-value part — verify fault before replacing |
| Condensate pump | Fits PCA/PKAA ceiling cassette models |
| Fan capacitor | Indoor or outdoor — match µF |
Technician Tips
Mini-split and VRF faults usually come from condensate drainage, thermistors, indoor-outdoor communication wiring, outdoor airflow, inverter protection, or control-board inputs. Match the code to the exact indoor and outdoor model, then check filters, drains, coils, wiring terminals, and sensor seating before replacing electronics.
DIY vs Pro: Cleaning filters, clearing drains, checking obvious loose connectors, and power-cycling once are reasonable first steps. Refrigerant work, inverter-board diagnostics, high-voltage testing, and sealed-system repairs need a qualified HVAC technician.
Need Replacement Parts?
Before ordering, match the HVAC part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
Pro tip: Mitsubishi Mr. Slim units store the last 4 fault codes in the PCB memory. On the wired remote PAR-31, navigate to CHECK > SELF DIAGNOSIS to view the fault history.