First off, it's a GlowWorm 18HXI ;)
Now has your plumber actually changed the thermistor - And - More importantly; has he changed the right one as there are two on this model...!
What is the fault code you are getting?
I'm not too sure why the pump was changed as this is a very easy component to test and prove... Without any further info and seeing the boiler it's hard to say; but in general with these kind of faults I go with:
-> Thermistor -> PCB -> Wiring Harness
But on saying that; he should check the temps in the pipes to ensure heat is leaving the boiler and that the thermistor is reporting the right resistances for the temps provided... GlowWorm have been kind and included a basic thermistor reading of 10K Ohms @ 25 Degrees C - From memory I believe they are NTC on this model.
Additionally; it is important with this model that you place a good covering of heatsink compound on the thermistor and that it is correctly sited otherwise you'll have no-end of problems...
But really do need to know the fault code if you want me to be more specific.
GasAid.co.uk, January 2007
