chaffoteaux flexiflue problem very weird

[exbg1]

am having a nightmare with a flexiflue. last month i replaced the pcb as it had burnt out. got a recon one from allspares in wembley. all was ok.
got another call last week. the fan had seized & the cooling ring had come loose on the shaft. fitted a new fan.
now the weird bit. the t/c would not hold unless i bridged out the connectors on the bottom of the pcb. (therefore t/c & thermo electric valve ok. spoke to ariston today & they said aps fault. what happens is that when i switch on the electrics the fan goes up to full speed & when i open the tap it goes back to rest speed. if i swap the black & orange wires on the microswitch (diaphragm) over then it behaves correctly but pilot will not hold unless i run the tap to get hi speed fan & then move the control knob from pilot to main gas. the microswitch is working correctly & so is the aps. also i fitted another pcb today but it made no difference so i think i can rule it out.
any advice please as i am quite stressed & so is the customer.
ps i am self employed gas safe registered & prior to working for 3 years on my own spent 18 years with british gas but left when they turned engineers into rip off salesmen !

 

[Boilerman2]

Check that the air inlet duct (Concentric flue) is clear we had a similar problem with one of these last year turned out to be a ton of leaves stuck in spiders webs reducing the air flow at idle causing the APS to drop out, which as you know knocks out the thermocouple interupter

 

[exbg1]

happens with the case off so not lack of air although i will check the flue anyway.
just cant work out why fan is on full speed at rest & goes to slow when microswitch makes when the tap is turned on.
have checked all wiring connections & all are as wiring diagram on boiler casing.

 

[exbg1]

boilerman2. just re-read your reply (thanks by the way) & don't understand one part. when you say the aps will drop out on idle & cause pilot outage, then what does the aps do? i thought it made on hi speed & went back to rest mode at low speed fan (idle)