Potterton promax combi he plus - error message E160?

Hi, I have a potterton promax combi he plus. It is showing error message E160 (fan wiring/ fan error).

At first the boiler would work, then error out on E160 intermittently. It would work if I reset it. Now, it has completely broken down. If I reset it, the fan goes for about 10 seconds then everything stops.

I have had two different gas safe engineers in to have a look at it including British Gas. The first replaced the PCB. The second replaced the connecting plug from the fan wire. Both got the boiler running temporarily for about 12 hours. Then it error red out with E160 again.

I am wondering:

a) What does a plumber need to do to get this fixed
b) Where I stand w.r.t paying the plumbers seen as neither has completed the work I asked them to do which is to fix the boiler.