P6

Mitsubishi Electric Air Conditioner

Severity: Moderate

What it means

Mitsubishi Electric Mr Slim P6 is the freezing/overheating protection of the indoor heat exchanger, documented on Mitsubishi Electric's Mr Slim error code PDF.
The official description: 'freezing/overheating protection of indoor heat exchanger working.'
Mitsubishi's published causes: 'short cycling of air cycle, dirty air filter, damaged fan, or abnormal refrigerant.'
P6 is mostly an airflow problem at the indoor coil — the most common single cause is a dirty filter that's restricted airflow to the point where the coil temperature drops below 0°C in cooling (or rises too high in heating).

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 with freeze sensors
Wall-mounted, cassette, ducted, and floor-standing PKA models all share P6
Mitsubishi's documentation: P6 = freezing/overheating protection of indoor heat exchanger working

Common Causes

Dirty air filter restricting airflow across the indoor coil
Indoor fan motor damaged or running at reduced speed
Return air grille blocked (furniture, decorative cover, dirty)
Refrigerant short charge causing the coil to run colder than designed
Outdoor coil dirty (in heating mode — affects heat exchange both directions)

How to Fix It

Clean or replace the indoor filter.

On wall-mounted PKA units: lift the front panel, slide out the two filters, wash with warm water (no detergent), let air-dry, replace.
On ceiling cassettes: drop the centre grille panel, slide out the filter, wash and dry, replace.
On ducted PEAD: locate the filter access (usually on the return duct or unit side) and replace per model.
A clogged filter is the #1 cause of P6 — this single step resolves the majority of cases.
Clear the return airflow path.

The indoor unit needs unobstructed return air.
On wall mounts: check that nothing is mounted in the 1-metre arc below the unit.
On cassettes: check the centre return grille is free of dust and there's no AV cabinet or furniture immediately below.
On ducted systems: check the return air vent grille on the wall or ceiling — vacuum dust off and confirm no rugs or furniture are covering it.
Check the indoor coil for ice.

If P6 fires repeatedly in cooling mode, the indoor coil may have iced over.
Power off the AC, then run it in fan-only mode for an hour to melt any ice.
You may see condensation drip from the indoor unit during this — that's normal ice melt.
Once the coil is clear, retry cooling.
P6 should not return if the airflow restriction was the underlying cause.
Confirm the indoor fan is running at full speed.

On the AC, set fan speed to High and turn on cooling.
Listen at the indoor unit — the fan should be clearly audible at full speed.
If the fan is slow or making unusual noises, the fan motor or its capacitor is failing.
That's a Mitsubishi service call.
Clean the indoor coil if filter cleaning isn't enough.

If the filter has been cleaned and airflow is unobstructed but P6 returns, the indoor coil itself may be clogged with dust that got past the filter over years.
A spray-can no-rinse evaporator cleaner (available at HVAC supply houses) can clean the coil with the unit off — follow the can's directions.
For heavy buildup, an HVAC technician with a coil pull and chemical wash is needed.
Schedule service if P6 persists after filter and coil cleaning.

If filter, airflow, and coil are all confirmed clean and P6 still appears, the refrigerant charge or the compressor is suspect.
This needs an HVAC technician with manifold gauges to check pressures on both sides of the system.
An under-charged system causes the indoor coil to drop below freezing even with good airflow — and a leak fix + recharge is the only resolution.

When to Call a Professional

Mitsubishi's published owner-side fix for P6 starts with airflow: clean the filter, clear obstructions around the return air, and confirm the indoor fan runs at full speed.
If those don't clear P6, the issue is on the refrigerant side and needs an HVAC technician with manifold gauges to check charge and identify any compressor or expansion valve issue.

Frequently Asked Questions

I cleaned the filter and P6 came back the next day — what now?

Three possibilities, in order of how likely they are.
One: the filter is the right type but only catches large dust — the indoor coil itself is now caked in fine dust that the filter couldn't catch.
That needs coil cleaning (step 5 on this page).
Two: the indoor fan motor has a worn-out capacitor and is running at reduced speed — the fan looks like it's spinning, but it's not moving enough air for the coil temperature to stay above freezing.
That's a service call.
Three: the refrigerant charge has slowly leaked over years and the coil now runs colder than designed for the same airflow.
That's also a service call but a more involved one (leak find + recharge by weight).