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.
Anyone else had a problem with this?