Mitsubishi Electric [P6] fault codewhat it means and how to fix it

P6 is the FTC stopping the heat pump because the plate heat exchanger, where refrigerant meets water, was about to freeze on the water side. The FTC installation manual calls it anti-freeze protection of the plate heat exchanger and gives a two-line action: see the action for L3, and check for the correct amount of refrigerant. The cylinder unit service manual explains when it fires: the error is detected when the refrigerant temperature drops rapidly during cooling or defrosting operation, judged by the refrigerant liquid thermistor or the pressure sensor. That is the picture to hold. Cold refrigerant enters the exchanger whenever the outdoor unit runs a defrost or, on units that can do it, cooling, and if the water on the other side is barely moving, the little water that is there gives up its heat and drops toward freezing. Ice inside a plate heat exchanger splits it, and a split exchanger puts water into the refrigerant circuit, which is as expensive a failure as a heat pump has.

So the FTC watches the temperature and trips first. The L3 checks, water leakage, strainer blockage and pump function, are all about restoring flow; the service manual’s own list adds low load or inlet water that is too cold, a defective outdoor fan control, a refrigerant overcharge or a clog in the circuit, the pressure sensor and a wrong capacity code, alongside leakage or shortage of refrigerant. That last one is the line that needs a qualified engineer. A circuit low on refrigerant runs its evaporating side colder than designed, and in defrost that cold side is the plate exchanger; Mitsubishi therefore tell the engineer to correct the amount, referring to the outdoor unit manual. PUHZ-W and PUHZ-HW outdoor units contain R410A and PUZ-WM and PUZ-HWM contain R32, both fluorinated gases, so checking or correcting the charge on those is F-Gas work by law; the PUZ-WZ Ecodan R290 range holds a flammable hydrocarbon that is not an F-gas and is trained-installer work instead.

One more thing the documents disagree on. The outdoor unit service manuals define P6 as "Freezing/overheating protection is working", with a heating-mode overheat branch, and Mitsubishi’s UK lookup heads its P6 row "Over heating at the heat exchanger", with lack of water flow, air, a wrong flow setting, a blockage or a pump problem as the causes. Either way the first checks are flow. Note that P6 is a P code in name only: the other P codes on the table, P1 and P2, are thermistor failures, and P6 behaves like the L family instead, which is why we cluster it with the flow and freeze protections rather than the sensors.