Potterton Profile 60e Main Burner cuts out

[pickse]

I have seen a few threads regarding these boilers but none seem to quite match my sympton.
Normally when the room stat or cylinder call for heat I see/hear the fan start, the pilot lights and then the main burner. Which I guess is normal operation.
But sometimes the main burner only stays lit for maybe a minute and goes out leaving the pilot lit?
I checked the solenoid coil volts and I have 230 on the blue common to both coils and 3 to 5 volts on the pilot (black wire) and main valve (white wire) other connection when the flames are lit. I guess this means they are switched on the neutral side of the coil?
Either way as the main burner goes out (leaving the pilot lit) the voltage on the white terminal rises to 230. I take this to mean that the pcb has decided to turn off the main burner?
I thought last week I had a pump problem or restiriction so replaced the pump and added a litre of X400 today but the problem is so far unchanged. Both up & down rad returns warm up reasonably quickly and the DWH return even quicker.
System is vented, 3 port valve with indirect HW cylinder. All seems to function properly when the boiler lights.
If anyone has any thoughts on what I could do next I'd appreciate hearing.

Thanks,
Steve.

 

[Puddle]

When main burner goes out when pilot light lit is fan still running
If so points to faulty main burner coil on gas valve...replace coil
if fan not running and pilot lit,points to faulty gas valve...replace

 

[pickse]

Thanks for your reply Puddle.
Yes fan keeps running and it stays in that condition for hours.
If I turn the boiler off, either boiler stat to 0 or mains off for a few minutes then switch back on the boiler will re-light and sometimes stay lit.
I see what you're saying about the gas valve coil but if the volts across the coil drop to 0 as the main burner goes out is this not indicating that the pcb has shut the burner off for some reason?
Is it normal for the main burner to be shut off leaving the pilot on.

Incidentally I took the fan out today because i thought I could feel a vibration. I found the fan has 1.5mm play fore and aft along the shaft and also two of the spring mounting washers are broken. So I'll change tha anyway.

Thanks again

 

[Puddle]

Sorry thought you said above there was power to coil
When pilot lit and burner out is ignition still sparking, if so maybe pilot to small for sensor to detect..clean out...check ignition/sensor lead, check earthling ok or could be pcb not sensing pilot light, this is not uncommon fault
if pilot lit and ignition off, most common fault would be coil on gas valve however if no power, would point to pcb,but unusual fault, check electrical connections, especially were plugs into board, also check for dry joint on power pin on pcb
It is a good boiler and will last you years once you get sorted out so why not treat it to a new fan, would think about replacing air pressure switch at same time

 

[pickse]

Thanks again.
Pilot look just like the picture in the book. Pilot lights on first spark and the spark stops immeadiately suggesting pcb know the pilot is alight. When the main burner goes out the pilot remains lit with no sparks and 230V across the pilot coil. (0V across the main coil).
I'll check the electrics as suggested and also treat it to an air pressure switch.
I have to agree is has been a good boiler. It is just coming up to ten years old and until now has given no trouble.
In fact the last couple of days it has been behaving perfectly normally.

Cheers,

 

[stevetheplumber]

Sorry thought you said above there was power to coil
When pilot lit and burner out is ignition still sparking, if so maybe pilot to small for sensor to detect..clean out...check ignition/sensor lead, check earthling ok or could be pcb not sensing pilot light, this is not uncommon fault
if pilot lit and ignition off, most common fault would be coil on gas valve however if no power, would point to pcb,but unusual fault, check electrical connections, especially were plugs into board, also check for dry joint on power pin on pcb
It is a good boiler and will last you years once you get sorted out so why not treat it to a new fan, would think about replacing air pressure switch at same time

i agree last of the good pottertons keep the system fernoxed and theres no reason it wont last another 15 years apart from the odd fan and electronics box

 

[RoryD]

If the fan sounds like it is surging or going on / off constantly it is the pcb. If the burner just drops out I would isolate electrics, remove and clean fan blades in 1/2 sink of water + old toothbrush then lubricate. If too much dirt on blades there is not enough torque to operate APS to start ignition sequence (sparking). Whe you've done that if still no spark then I would test / replace APS

oops!! just re read symptoms sorry bout mis info, solenoid GV2 on valve is knackered...test resistance..there may be voltage but if it ain't gettin thru the coil we all stay cold

 

[pickse]

Thanks for the suggestions.
I had already ordered a fan so fitted that three days back.
I also repluged the connections on the pcb and had a good look at the APS tubes etc.
I have to say the boiler has performed faultlessly since.
I don't know how sensitive the APS is but with the old fan the vibration I could feel was causing the front cover the move. Not sure if that was enough to upset the pcb? but as an electronics man I'm more comfortanble with the idea of a bad connection or dry joint on the pcb which I've made good again (if only for now).
Anyway it's nice to come home to a warm house.
Thanks again for the help.

Steve.