However, the problem started occurring again and so I booked an appointment to have a look at the problem myself as the other engineer was on holiday. The next day the F3 fault had escalated to an F1 fault and the boiler was failing to light so I attended the same day. I looked at the boiler and it was a sparking but failing to light, but there was no problem with the gas supply. I realised that the F1 code was due to the overheat thermostat cutting the boiler out and not allowing the boiler to light. I reset the overheat thermostat and the boiler was able to light again.
Running the boiler for the hot water, the boiler got up to temperature quickly and cut out before the hot water was hot at the tap. The hot water would then got quite hot and then the boiler would relight as the boiler cooled down. The water would cool before the boiler could catch up. The boiler was also quite noisy suggesting that the circulation was poor. There didn't feel like there was much vibration coming from the pump. I removed the central screw on the pump to check that the spindle was free. When the boiler was running the pump was spinning but it didn't look as fast as it should be.
Fortunately the customer had a replacement pump and housing in his garage. When changing the pump I changed the fibre washer on the top connection as they don't seal very well when reused. I also had to clean the brass spindle that the pump slides onto as there was a fair amount of scale build up on it which was preventing the o-ring from making a good seal.
Replacing the pump solved the F1 and F3 errors and the boiler was running well although the hot water was still quite hot it was modulating well.
The works described in this blog are a record of work undertaken and should not be taken as guaranteed to solve your problem.