Welcome to the forums @spacebiscuit. I’ll reach out to Mitsubishi for advice. 

As an aside, have you reached out to your original installer?

@spacebiscuit I've heard back from Mitsubishi.

They've informed me that for error codes they have a platform on their website that initially identifies what the issue could be – https://les.mitsubishielectric.co.uk/installers/error-code-lookup – and as you've said it's a thermistor fault. 

For any technical queries, they've suggested you contact the installer first, but failing that you could contact their home helpline: 

Ecodan Homeowner Helpline
0161 866 6064
Opening Times:
Mon - Fri: 08:00-17:00
Sat: 08:00 -12:00

Thanks yeah one of those Thermistors - I'm trying to figure out if my control panel is able to tell me which one exactly - a Youtbe video shows a guy accessing menu options to find exact details but my unit looks older!

@spacebiscuit I'd definitely recommend calling Mitsubishi so they can walk you through it. If anything, it'll point you in the right direction. This is quite a specific error that most homeowners won't have come across, so if you do get answers, it would be great if you could post your outcomes here.

I spoke Mitsubishi on the phone - they just read back to me what I can find online. they want £292 just to come and have a look 🙂

@spacebiscuit, ouch. I can understand why there may be some diagnostics required based on the error. I'll reach out to my installer network to see if a board replacement is the solution.

Posted by: @spacebiscuit

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I spoke Mitsubishi on the phone - they just read back to me what I can find online. they want £292 just to come and have a look 🙂

You will probably find that the thermistors are small metal cylinders, about the same diameter as a pen, and probably several centimeters long, with a cable coming out of one end.

If you look at the rear of the outside heat pump unit you should be able to see the thermistor measuring the outside air temperature.

The actual thermistor element is located within the metal cylinder, which provides both physical protection and thermal conductivity. Unless the metal cylinder has been physically damaged then the most probable cause of malfunction is damage to the cable or a loose connection. I think on the Ecodan the thermistors are connected to the circuit board by plugs and sockets, so I would suggest checking these for a loose connection.

If you can supply the model number for your unit I may be able to download the relevant manual.

Thanks - over the weekend I order a replacement power board which I switched out - didn't fix the issue. However after checking the thermistors for their resistance readings the pump spun up after reconnecting them to the board - the pump ran for 30 minutes long enough to heat the cylinder. Then an hour later I had the same U4 error again. I left the machine for sometime, removed each thermistor form the board took the reading and again the pump spun up, this time for about 5 minutes but stopped when I was carefully removing the corner panel where the PCBs are.

Whenever the pump stops I get good resistance readings, I'm wondering if it is an issue on the control board rather than the actual thermistors?

Posted by: @spacebiscuit

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Thanks - over the weekend I order a replacement power board which I switched out - didn't fix the issue. However after checking the thermistors for their resistance readings the pump spun up after reconnecting them to the board - the pump ran for 30 minutes long enough to heat the cylinder. Then an hour later I had the same U4 error again. I left the machine for sometime, removed each thermistor form the board took the reading and again the pump spun up, this time for about 5 minutes but stopped when I was carefully removing the corner panel where the PCBs are.

Whenever the pump stops I get good resistance readings, I'm wondering if it is an issue on the control board rather than the actual thermistors?

I would suggest that you check the soldered joints on the board with a magnifying glass. Intermittent faults can be caused by 'dry joints' on a circuit board.

Also check the wiring for damage or bad connections in the plugs and sockets.

I found this on the board where TH4 (Thermistor Discharge) plugs. The rest of the solder joints look clean as can be seen on TH3.

Does the joint look bad? 

Posted by: @spacebiscuit

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I found this on the board where TH4 (Thermistor Discharge) plugs. The rest of the solder joints look clean as can be seen on TH3.

Does the joint look bad? 

Try moving the plug/socket and see if there is any movement or that the alarm clears or appears.