Mitsubishi 'UF' fault code

[jammy23]

I have been working on a Mitsubishi PUHZ-RP125YHA2 cassette system today, original fault on the unit was a P5 fault which I traced to a blocked drain and very dirty tank pump. Cleared the problem which seemed to clear the P5 fault, when the unit seemed to be running ok I checked on it before leaving site and the fault had switched to a UF fault.

I know that this has something to do with compressor overcurrent/compressor lock, the unit seemed to run fine after the drain was cleared. I isolated the indoor unit by the isolator on the unit and when I switched it back on it came up with E9 I think which was communication between indoor and out door. I couldn't get this fault to clear so isolated both units by their isolators, switched on outdoor then indoor and that fault cleared and unit seemed ok but then decided to through up this UF fault.

I have checked the outdoor unit and watched it try and run but the compressor seems to try and run then shuts down again either as if trying to switch to run but cant or there isn't enough power feeding the compressor so it is shutting itself down.

I don't work too much with Mitsubishis of this type so am a bit stumped, could just do with some ideas of what to look for etc. I have tested terminals on the compressor and they all look good but with there being like 4 boards on the unit and no wiring diagram it would be great to have an idea of what could be causing the compressor to run intermittently like it is doing.

I have also tested one of the capacitors (which tests good) but am unable to get to the two in behind the vertical control board to test those as I thought it could be one of them not supply enough of a kick??

[temperzone]

I would check the output of the power supply of the board. The one going to the compressor (U,V,W), you may have lost a phase. Also check voltage between P & N on this board, you should get around 320V if not I'd change this board..Good luck

[jammy23]

I spoke to Mitsi technical on Friday, I managed to find a wiring diagram online which showed the power going through the IPM before going to the compressor so I thought testing the power coming in to the compressor would be best way. They basically said anything less than 100v down any of the lines and the inverter board is faulty....when tested it was only getting about 30-35v down all three lines so looking to swap the inverter board.

Thanks anyway!

[beezeraircon]

How did you go on with the uf fault? did changing the board help?
I have the same fault, changed board and same thing, ive discussed this with Mitsi and they think this is power supply.