Boiler continually stays on - Ariston Eurocombi A23

[Theydonites]

Hello,

My Mother in laws boiler has developed a fault and rather than calling someone out I thought I would look online first to try and establish if I can diagnose and fix the fault (fingers crossed!)

The Boiler is an Ariston Eurocombi A23 which is approx 8years old. For some time now the boiler can only be used if switched off and on from the main switch to the boiler otherwise it stays on all the time, it continually starts up and then shuts down. In 'summer mode' (hot water only) the boiler does not fire up when hot water is called for. The only way that the boiler will fire up and provide hot water is if its in 'Winter mode' but when it is switched to winter mode the radiators come on as well.

I've replaced the diaphragm in the diverter valve and I have also replaced the temperature probe, temperature sensor which attaches to one of the pipes. The pin from the diverter valve goes out when hot water is called for.

There are two PCB's in this boiler, one situated on the left and one on the right. The one on the left appears to control the temperature of the water and rads (there is an external room thermostat which runs back to this) the PCB on the right has mostly relays and seems to control when the boiler switches on an off. When I checked this board I could see that one of the resistors had corroded so I replaced this but the fault still remains.

I've spoken to the manufacturers who seem to think that either the diverter valve or one of the temperature sensors maybe be faulty but I have replaced these already. I've got a niggling feeling that its the left PCB but I've got no way of checking this.

Have you got any advice/guidance please? My mother in law doesn't want to pay for a new boiler to be installed as she will be moving out in a year or so to sheltered accommodation. I've offered to pay for an engineer to look at it but if its something simple I'm hoping I should be able to fix it.

Many thanks