Drain pump
Common dishwasher and washer water-removal part.
Mitsubishi P4 Error Code — What It Means
Mitsubishi error code P4 indicates a drain system fault. The indoor unit has detected water not draining correctly from the condensate pan. On wall-mount and cassette units, this usually means the drain line is blocked, the float switch is stuck, or the drain pump can’t move water.
Common Causes
- Blocked drain line — Algae, slime, or debris is restricting the condensate hose.
- Stuck float switch — The overflow float is jammed in the high-water position.
- Drain pan full of sludge — Dirt from the evaporator has built up in the pan.
- Failed drain pump — On cassette or ceiling units, the pump has stopped lifting water.
Step-by-Step Fix
- Turn off the unit and remove power.
- Inspect the drain hose outlet. Use a wet/dry vacuum to pull debris from the hose.
- Open the indoor unit and inspect the drain pan for standing water.
- Clean the drain pan and verify the float switch moves freely.
- If equipped with a pump, energize the system and confirm the pump runs when the pan fills.
- Reassemble, restore power, and run cooling mode to verify proper drainage.
Parts Often Needed
| Part | Notes |
|---|---|
| Drain hose | Replace if kinked or brittle |
| Float switch | Model-specific indoor unit part |
| Drain pump | Required on many cassette and concealed units |
Technician Tips
Record the full model number, exact code, and operating mode before replacing parts. Similar code labels can point to different sensors, pumps, boards, or wiring circuits across product families.
Reset once after correcting an obvious cause. If the code returns immediately, leave the equipment off and test the named circuit instead of repeatedly cycling power.
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.
When to Call a Pro
If the drain pump is not operating or the unit is built into a ceiling with poor access, a Mitsubishi technician can diagnose the float circuit and pump wiring safely.