View Full Version : Dreaded P8 Error on Mr Slim unit

Vic Smith
25-11-2016, 12:46 PM
Hi all, wonder if you can help me at all please ?

I have a Mitsubishi Mr Slim with a PUH-5YK outdoor unit. This works really well (cooling mode) when the weather is not too hot (up to 23 or 24 degrees C) but when the mercury hits 30 or so, the compressor and fan starts up for a couple of minutes, stops for approx 3 or 4 minutes, then repeats, until a P8 error finally appears.

The outdoor unit is one of the earlier ones with the 8 leds. Led 7 is lit indicting that overheat protection has been triggered, and the manual indicates that the thermistor is broken, or that the coil temperature is over 67 degrees C.

The coil temp is able to be 'converted' from the LED's if SW3-2 is set on. This gave a total of 216, which should convert to 67 degrees C according to the graph in the manual for the indoor unit.

Next step was to confirm that the thermistor was working as expected, which is where I ran into a bit of trouble. I had been led to believe that there were 3 thermistors in the outdoor unit, however I was only able to find one. The one I did find provided ohm meter readings consistent with a start temperature of 28 C (ambient outdoor - 4.58 k ohm)) rising to 36 C at the point of cut-out (3.65 k ohm). AFP (Affirmative Finger Probing) confirmed a small rise in temperature at the pipe itself, and definitely NOT 67 C.

I have had no luck at all in locating the appropriate manual for the outdoor unit, and am assuming that the manual for the indoor unit is correct (PEH-5EK ?).

Is there any chance of someone confirming for me:
- That the steps I have completed and conclusions drawn so far are correct !
- That there is only one thermistor in the outdoor unit
- The the 'conversion' that I did for the thermistor is correct please ?

I suspect that if they are, then I will need to replace the board .....

A link to the correct service manual for the outdoor unit would also be appreciated if anyone has one please ?

Sorry about the length of the post ...

Many Thanks,


25-11-2016, 01:28 PM
Hi, Vic!
The manual is here
The thermistor resistance value at 67C is 1.2kOhm.

Vic Smith
25-11-2016, 11:27 PM
Thanks Narcom, the manual certainly helps to confirm that I have the thermistor correctly identified.

Now to see if I can fix this !

26-11-2016, 07:53 AM
You are welcome!
You can also find on Google Play the app. "ME Engineer" and use the Thermistor Value Lookup feature.