air pressure switch sequence

[newbie1]

have been to boiler breakdown potterton netaheat and pump runs/fan runs but boiler wont fire
now aps is 3 wire
common/nc/no
with no call for heat or water the aps is showing continuity between common and nc and also between nc and no
i am erring towards a pcb fault as i believe with no call for heat and no fan running the aps should not have continuity between nc and no
tested aps with leads off and continuity between common and nc and no continuity between nc and no so would seem aps is fine until leads re connected
ideas appreciated

 

[migoplumber]

dont know the boiler. try joining the wires from c to nc (with the power off) that eliminates the pressure switch. more likely to fail than your pcb.

 

[Puddle]

The three wire aps allows boiler to check all is in correct operating mode before boiler start up and once aps activated that fan is working,your tests suggest conntacts ok,but when live maybe getting slight back feed or just normaly through board
It could be pcb but unusual fault at this stage of start up
Disconect air tube to aps and manually blow down to see if switches relay over on pcb to power up boiler
On this boiler due to age could be fan running slow and worn,stiff apr,if one gone,faulty best to replace both items

 

[Rusty]

Blow very gently though and you should hear it operate, blow to hard and you'll break the switch anyway and won't prove nothing
I was always taught and have found, every time you think its the pcb its something else, the one time you are sure its something else, it will be the pcb, good hunting

 

[newbie1]

aps is def working correctly when not connected to the 3 wires in that common to nc is continuity
and nc to no is no continuity
but as soon as i connect the wires with no call for heat/water and pump and fan not running the aps shows continuity between the nc and no terminals and my understanding is that for ignition seq to commence the circuit must be at rest between nc and no and only goes to continuity between nc and no when fan blows