Worcester 350RSF reset overheat button fault

[topdog]

Hi everyone,, working on a worcester 350 RSF, and customer is having to press the overheat thermostat reset button, on most days.
But everything is running ok now i'am here, so finding it hard to detect a problem, customer said they sometimes have to bang on the boiler
for it to run, I have checked for any loose connections, all are fine.
ANY IDEAS........

Overheat thermostat change ???
Sticking Pump ???
Heat exchanger blocked up with sludge ???
thermistor problem ???
Pressure switch fault ???

Like I said boiler running ok at the moment.

Any positive suggestions would be a appreciated.
Many Thanks

topdog

 

[DirksPlumbing]

On a 350 my first guess would be the(a) pump. So it does happen in HW mode I take? How is the pressure in the boiler?

 

[arran197]

Overheat stat. It'll stop resetting soon too. I've seen a few old Worcesters do this.

 

[Billy Bob Bob]

overheat stat first guess replaced loads,pump second shout