Mitsubishi Mr Slim P4 Error — Drain Sensor Fault Fix

What it means

Mitsubishi Electric Mr Slim P4 is the drain sensor fault documented on Mitsubishi Electric's Mr Slim error code list (mitsubishielectric.com.sg).
The official description: 'drain sensor (DS) abnormal — open or short circuit.'
The drain sensor is a thermistor that monitors the condensate drain water in the indoor unit's drain pan.
When the controller can't read the thermistor (it's open-circuited, short-circuited, or its connector is loose), P4 fires and the AC shuts down.

Affected Models

Mitsubishi Mr Slim P-Series indoor units (PCA, PEAD, PKA, PLA, PCH)
Mitsubishi Mr Slim K-Series indoor units (older lineup)
Mitsubishi City Multi VRF indoor units (PEFY, PMFY, PLFY, PKFY series)
P4 specifically appears on indoor units with a drain pump and drain thermistor
Mitsubishi's documentation: P4 = drain sensor open / short circuit

Common Causes

Drain thermistor connector loose at the indoor PCB
Drain thermistor wire damaged inside the unit (rodent, install error)
Thermistor itself failed (open or short)
Indoor controller board defective and misreading a healthy thermistor
Recent service work left the thermistor disconnected

How to Fix It

Power off at the breaker.

Turn off the indoor unit at its dedicated breaker.
Wait 5 minutes for capacitors on the indoor controller to discharge.
Mitsubishi's electrical service requires full isolation before opening the electrical compartment.
Open the indoor unit's electrical compartment.

On wall-mounted PKA units: remove the front panel to access the PCB.
On ceiling cassettes (PLA): remove the centre grille and the controller cover.
On ducted (PEAD): the controller is accessed via a service panel on the unit's side.
Locate the indoor controller board.
Identify the drain thermistor connector.

Mitsubishi labels the drain thermistor as DS (sometimes TH3 on older units).
Find the DS connector on the indoor PCB.
Confirm it's seated firmly — re-seating a partially-loose connector is the single most common P4 fix.
Test the drain thermistor resistance.

Disconnect the DS thermistor from the PCB.
With the unit at room temperature, measure resistance across the two thermistor pins.
Mitsubishi's Mr Slim service manual lists expected resistance at given temperatures for the specific model — compare your reading against that chart.
What's unambiguously bad: if the multimeter shows open circuit (infinite resistance) or short circuit (near 0 ohms), the thermistor is failed and needs replacement, regardless of the model-specific spec.
Trace the thermistor wire for damage.

If the thermistor itself tests okay at the connector, the issue is in the wire between the thermistor and the PCB.
Follow the wire from the connector toward the drain pan.
Look for crushing, kinks, or chew marks.
Replace any visibly damaged section of wire.
Replace the drain thermistor or controller board.

Mitsubishi sells the drain thermistor as a service part — get the part number from the service manual for your specific indoor model.
If the thermistor tests good but P4 persists after re-seating connectors, the indoor controller board itself has failed.
The board is more expensive than the thermistor — try the thermistor swap first.
Both replacements are Mitsubishi-trained-tech work.

When to Call a Professional

Mitsubishi's published P4 troubleshooting is: measure thermistor resistance at known temperatures, compare to the Mr Slim resistance chart, and replace either the thermistor or the indoor controller board.
This requires opening the indoor unit's electrical compartment and using a multimeter — that's HVAC technician work, not owner work.
If the unit was working fine and P4 appeared suddenly, a loose connector after a service visit is the most likely cause.

Frequently Asked Questions

Can I just ignore P4 and keep using the AC?

No — Mitsubishi shuts the AC down on P4 by design.
The drain thermistor monitors the condensate level; without it working, the AC can't detect a flooded drain pan.
If the AC kept running with a P4-faulty drain thermistor and the drain happened to back up, water would overflow the pan and damage your ceiling (on ceiling cassettes) or the wall and floor (on wall-mounted units).
P4 is a protection — the AC stops itself before water damage can happen.
Get the sensor fixed; don't bypass the fault.