Halstead ace high will not reset?

Hi,
I have just serviced a Halstead ace high 30kw combi for a customer. I closed the gas valve to check the safety device and the boiler went to lockout as expected. When I tried to reset it, and the overheat stat, it would still not reset. I have turned it off and left it for 20mins with still no luck. I even disconnected all the leads from the PCB with no luck still. Green power light on and green lights on in ch and dhw mode but still lockout. Microswitches seem to be working ok. Any ideas would be appreciated as Halstead seem to have given up. Cheers