Potterton Suprema 40 short cycling?

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Hi all. I have a Potterton Suprema 40 boiler which is misbehaving. It turns off and on again once every few minutes, even when room stat is calling for constant heat. Then once every little while it’ll lockout - always in the morning, sometimes precipitated by servere pipe knocking all around the system. Once reset it’ll fire up, but the short cycling continues. I’ve done a bit of reading but the other threads don’t seem to cover these problems. Does it sound like the PCB problem? Or overheat stat maybe?
 

CBW

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Does it lockout as an overheat situation? Is there water in your feed and expansion tank?
 
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Hi Chris. I believe it does lockout as an overheat. If I press the reset for 10 secs to reveal the code, the red light changes to 5 flashes then a pause (from memory) which I think is overheat? Only happens when firing up (or attempting to) in the morning. Haven’t checked the expansion tank but I will
 
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Hi all. Thought I’d pull the cover on the boiler today to see if anything obvious was at fault. I did notice the thermistor pocket was completely void of any conductive paste, and the element at the end of the thermistor was looking a little corroded. Could this be the issue/ cause of the constant turning on/off every few minutes? New one is now ordered regardless as they’re v cheap. I went in the loft and checked the tank - all seems fine
 
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