Warning: Disconnect power before servicing.
ErrBase

Mitsubishi Mini-Split P4 Error Code: Drain Pan Overflow

Quick Answer

Mitsubishi P4 means the drain pan is full of water. The condensate that drips off the evaporator coil during cooling cannot drain away. This is almost always a clogged drain line - typically a buildup of algae, mold, or debris in the drain pipe.

Mitsubishi P4 means the drain pan is full of water. The condensate that drips off the evaporator coil during cooling cannot drain away. This is almost always a clogged drain line - typically a buildup of algae, mold, or debris in the drain pipe. Clearing the clog is the fix in most cases. Pour water down the drain line or use compressed air to clear the blockage.

MitsubishiMinisplitSeverity: high88% DIY Success
Time to Fix
15–45 min
Difficulty
beginner
Parts Cost
$8 – $15
Tools Needed
Wet-dry vacuum, Compressed air can

What Does the P4 Code Mean?

P4 fires when the drain pan is full of water. Clear the drain line blockage. On units with gravity drainage, blow compressed air backward from the drain outlet. Then verify the drain line has a proper downward slope toward the outlet.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Drain line clogged with algae or debris40%
Drain slope incorrect (not enough fall)24%
Float switch stuck in tripped position22%
Drain pump failed (if one is installed)14%

Symptoms You May Notice

  • Indoor unit shows P4
  • Water dripping from the indoor unit onto the wall or floor
  • Visible water in the drain pan under the unit
  • Unit shuts down on hot humid days

Can you reset a Mitsubishi minisplit to clear the P4 code?

After clearing the drain line: empty any standing water from the drain pan with a towel or wet-dry vacuum. Then turn off the circuit breaker for 60 seconds and back on. Power on the unit with the remote. The float switch should reset automatically once the pan is empty and P4 will clear.

Tools Required for Diagnosis

Wet-dry vacuumCompressed air canTowelsSmall funnel (for pouring test water)

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

1

Turn off the unit immediately when P4 appears

Turn off the unit immediately when P4 appears. Standing water in the drain pan will continue to drip if the unit runs. Locate the condensate drain line outlet - typically a small tube exiting through the wall to outside.

2

From the outdoor drain outlet

3

Test the drain line by slowly pouring one cup of water directly

4

Check the drain line slope

5

Reset the unit by cycling the circuit breaker for 60 seconds

Replacement Parts

If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.

Part Name
Condensate Drain Pan Biocide TabletsN/A - any HVAC condensate biocide · $8–$15

Repair vs. Replace Calculator

Appliance Age (Years)7 Yrs
Labor Approach
Our Verdict
REPAIR
Estimated Cost: $12 (Parts) vs $1900 (New Machine)

Frequently Asked Questions

How do I prevent Mitsubishi mini-split P4 errors?
Clean the drain line every 6 months by pouring a mixture of equal parts white vinegar and water through it from the indoor unit side. Drop a biocide tablet in the drain pan quarterly. In high-humidity climates where the unit runs constantly, monthly maintenance of the drain line prevents most P4 occurrences.
Can a dirty air filter cause a P4 error?
Yes, indirectly. A very dirty filter restricts airflow over the evaporator coil, causing the coil to get colder than normal and produce more condensate than the drain line can handle. Clean the air filter first as part of P4 diagnosis.
Is it safe to run my Mitsubishi mini-split with a P4 error?
No. Running the unit with P4 active will cause the drain pan to overflow and water to spill inside the wall, causing mold growth and structural damage. Turn the unit off and fix the drain before resuming operation.
My Mitsubishi shows P4 only on the hottest days. What does that mean?
The drain line has a partial blockage that cannot handle peak condensate flow. On mild days the flow is slow enough to trickle through the partial clog. On peak summer days with maximum dehumidification, the clog cannot keep up. Clear the drain line completely rather than partially.
After clearing the drain, how do I know the float switch reset properly?
With the unit powered off, carefully open the indoor unit front panel and look at the drain pan. The float switch should be in the lower (resting) position. If it stays elevated even with the pan empty, the float is stuck and needs to be manually lowered or cleaned.

Related Mitsubishi Minisplit Error Codes

Code: HUBMitsubishi Mini-Split Error Codes: Complete MSZ and MXZ Series GuideCode: E9Mitsubishi Mini-Split E9 Error Code: Drain Pump MalfunctionCode: P5Mitsubishi Mini-Split P5 Error Code: Drain System MalfunctionCode: RESETMitsubishi Mini-Split Error Codes: Complete MSZ and MXZ Series Guide

Same Fix on Other Brands

LG CH52:

Models Known to Experience P4 Errors

This repair applies to most Mitsubishi minisplits with this error code. Common model numbers include:

MSZ-GL12NA, MSZ-FH12NA, MSZ-FS12NA, MXZ-2C20NAHZ

Last verified for technical accuracy on March 14, 2026