I have a potterton suprima 60 boiler which over the last 2 weeks has
gradually got worse with the locking out problem. I've had the PCB out
, fan, air pressure switch ( which i have tested for continuity)
cleaned all contacts. Everytime I have a look at the innards of the
boiler( 3-4 times so far) it works perfectly for 24-36 hours before
starting to lock out more and more frequently until it goes straight
to lock out after a few attempts at lighting. At the moment the boiler
ignites and burns for a few seconds but the the green light continues
to flash at high frequency instead of going on continuously as it
should , the burner then extinguishes. According to the fault finder
flow chart the problem is either with the PCB or the gas valve,
depending on whether there is 300vDC between red and black wires to
the gas valve. I need to get a volt meter to check this. The PCB is a
reconditioned unit from CETltd which I purchased last Winter after
similar lock out problems although last years problems were sorted out
with a new air pressure switch. When I took the PCB out I noticed some
scorch marks around some of the resisters on the middle of the board.
My gut instinct is to replace the PCB again as this seems to be the
common fault with these boilers. It seems like the control is not
getting a boiler has lit signal and so switches off the gas valve. Any
suggestions gratefully received.

Jim