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 {#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 | Amazon | Replace if kinked or brittle |
| Float switch | Amazon | Model-specific indoor unit part |
| Drain pump | Amazon | Required on many cassette and concealed units |
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.