Glowworm Micron 50FF, software error?

We have a Glowworm Micron 50FF installed. It was not working, apparently a fan problem. Engineer changed fan to a new one, all worked for about a day, then switched off (reset light on, burner light flashing 2Hz - software error). Switched on and off couple of times, came back on in a day, then off again.

Now every time you switch it on, it tries to burn a bit and then goes into reset light on, burner light flashing state (software error according to manual).

The technician that came was not sure what fault it is, made couple of calls came to conclusion that it is most likely thermistor, but may be PCB board as well.

Anyone have had a similar experience? Any advice?

Kind Regards,
D