Worcester 24CDi RSF boiler

[littletoe46]

A most peculiar fault with CH : can anyone help? Boiler is about 6 years old and in VGC
DHW works perfectly. CH operating pressure about 1.75 bar. PROBLEM ... CH comes on and reaches set temp. Gas goes off, pump etc continues to run. As rads cool the gas will not fire up again, even if rads become cold, or the CH stat is turned up. If ANYTHING is done that will fire up the burner again e.g open DHW hot tap / re-set CH timer / turn boiler off then on again/ or (works sometimes)press re-set button. The CH warms up again and the cycle repeats itself.
New thermistor fitted on last service quite recently. My service engineer is baffled and suggested calling in Worcester, but this is an expensive option.

[Atticus Finch]

What is each light doing when the fault occurs? on or off or flashing - slow/fast?

[littletoe46]

None of the fault lights are coming on at all. I've checked the manual against possible faults, but this one is not covered.The only lights that come on is the Burner On light when the boiler is running, and the CH On light.( and the DHW light when required) Naturally the Burner On light goes Off when set temp is achieved. Burner stays off when CH temp reached, CH On light stays on. The Burner on light only comes on again if anything at all is done to re-set boiler or DHW is turned on. Once Burner On is re-established, the gas stays on with the CH until set temp is reached again. If nothing is done to Re-set burner on, it stays off and rads gradually get cold.

[Atticus Finch]

So when the fault occurs the green light is ON, but all red lights are OFF?

[htg engineer]

Probably a faulty sensor/thermistor, it may be a sensor/thermistor with alot of furring/limescale, this will cause the heat to be maintained and this wont allow the boiler to fire up again. You need a multimeter to check the sensors - or just replace them as they're probably the cheapest part that could be at fault.