Had a problem with boiler locking out after a minute or so on CH. Boiler OK on HW.
Problem seemed to be thermistor so it was changed. Boiler was a bit better but not right. After a good deal of head scratching I had a good look at the thermistor. It is a clip on type with the printed element sandwiched between two copper foils. I removed the top foil and filled the area with heat sink compound (from Maplin). Re-fitting with more compound on the pipe and all is now well.
The thermistor can only work accurately if the heat source and the element are reliably thermally connected.
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