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I have an Air Source Heat Pump that has stopped working, I have an error code which is:

Faulty thermistor - dead short/open circuit.

There are 5 thermistors in total, one built into the power board which I have to replace and 4 external which attach to another board:

enter image description here

The thermistors plug in at the top of the board is indicated by the 4 arrows. I used my component tester and all of them are read as resistors — does this indicate that they are not faulty?

Anyway, I noticed that the blue resistor also indicated by a purple arrow looked as if it was burned — see the actual photo below. Whilst on the board I tested it, and it was reading as a capacitor.

enter image description here

So I removed one leg of the resistor and tested it again. This time it was read as a resistor correctly, so I soldered it back onto the board. Plugged everything back in the heat pump came to life without the error. It ran for about 10 minutes, before it stopped and the error code returned. Also, when I test the resistor, it again reads as a 33 pF capacitor.

I'm a little confused, the thermistors are seemingly OK. Is that resistor at play, or just a coincidence that removing and resoldering the leg fixed something temporarily?

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If you are testing a resistor in circuit you are not reading the resistor, you’re reading the resistor and whatever it’s connected to. You don’t say that you measured the blue resistor after resoldering it but before running the unit again, so we can’t tell if the the reading you got after the failure is different than it would have been before.

Saying that something ‘reads as a resistor’ isn’t of much help, you need to know what resistance it reads and what resistance it’s supposed to be. A thermistor may read some resistance, but is it the correct resistance for the temperature?

As for disconnecting and reconnecting the one resistor, that probably did nothing. It’s more likely that just having the unit powered down for a time caused it to start up without the error. It may be that the error occurs after something heats up, or it could just be that the micro took some time to determine there was an error.

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  • \$\begingroup\$ Understood regarding testing in circuit. I should have added - I'd been trying to start the pump throughout the day after I had fitted the new power board and if flat refused to start - other than the one occasion as mentioned. Although I agree I have to find the actual resistance reading the thermistors should give, I'm just wondering if a resistor giving inaccurate resistance would be detected as "dead shot/open circuit" - which would appear to be a dead component rather than a poorly functioning one? \$\endgroup\$
    – Alan A
    Commented Jun 15 at 23:23
  • \$\begingroup\$ @AlanA Possibly. Hard to tell without knowing more about the circuit and unit’s self test function. We also don’t know how you are testing it, it sounds like maybe you are using one of those component identifiers rather than a DMM, is that correct? \$\endgroup\$
    – GodJihyo
    Commented Jun 16 at 1:04
  • \$\begingroup\$ Yes it is a component identifier, it also tells me the resistance of those Thermistors. Today after the machine had been off over night I took the readings and noted them down - they all seems to be within correct limits. I then plugged them all back in, and again the machine started to work - for about 5 minutes before powering down again. I'm not sure if taking the reading resets something, albeit temporarily. Can thermistors fail gradually? \$\endgroup\$
    – Alan A
    Commented Jun 16 at 22:08

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