Short cycling in Potterton Kingfisher

Hello,
Our boiler is an old (installed in 2005), Potterton Kingfisher (MF 40-100), using a pressurised system. It recently started short cycling, where the pump runs for a few seconds and then the green LED Flame indicator lights and a couple of seconds later both the pump and the LED stop. This cycle then repeats endlessly. Upon checking I discovered that the system pressure had dropped to almost zero. So, I repressurised the system to 1.5 Bar. But the short cycling persisted. Earlier today I removed the controller PCB and the Air Pressure Switch from the boiler. The PCB looks pristine, no scorch or burn marks (a new original PCB was installed around 8 years ago, replacing a failed unit). The fuse conducts, the coil resistances of the relays match their datasheet values. I forgot to check the two transformers, but they both appeared undamaged. I then checked the Honeywell Air Pressure Switch. It works as it should, opening the (normally closed) Common to NC connection, and simultaneously closing the (normally open) Common to NO connection when I briefly & gently sucked air out of the 'L' gas inlet.
So, my question is, given the above information, what could be the likely cause of this short-cycling?
Many thanks in advance!

Hello,
I couldn't find a way to edit my original post, so am typing this new post. In my original post, I wrote "couple of seconds later both the PUMP and the LED stop. This cycle then repeats endlessly." I actually meant the FAN. So, the fan is working. Next, to check if the Grundfos water pump is also working, I pressed one end of a 50cm steel rod to the pump body and put my ear to the other end. I could hear a continuous whirring which was independant of the sounds made by the boiler (ie it didn't start/stop in sync with the boiler sounds but stayed on all the time). Which is what the pump should do. So, we now know that both the fan AND pump are working.

I have attached the Service & Installation manual of the Potterton Kingfisher MF to this post. I've also attached a photo of the Controller PCB which I took when I examined it earlier today. As you can see, its pristine.

On page 31 of this manual is given the Pictorial Wiring Diagram. I am trying to eliminate those parts which could possibly shut down the gas valve, since this is the fault I'm trying to eliminate. First, right at the bottom of this diagram, does the greyed lead to the temperature sensor mean the this part is an alternative to the Overheat thermostat, or are they BOTH connected? When I opened the casing today to reach the controller, I noticed the back of the Overheat thermostat, with leads attached to the round black body and white (presumably thermal conduction) caulk surrounding this body. This was placed roughly in the middle of the boiler, just underneath the cover panel. Is this thermostat a thermistor? How can I test to see that its functional? If its a thermistor then measuring its resistance at room temperature would tell if its okay or not.

Just to reiterate, I can hear the "Whump" of the gas igniting immediately after the Flame LED lights up, followed a couple of seconds later by the shutdown of the LED, the sound of burning gas & the fan. This then repeats: Fan switches on, around 5-10 seconds later the Flame LED lights, a couple of seconds later the gas ignites, a couple of seconds later, all of these shut down.

EDIT: I just thought of this. Is it possible that the fault is due to one (or more) of the non-latching relays having its (their) contacts welded together (thereby being ON continuously)? Earlier I checked that the relay coils were okay, but I never thought of checking the Normally OFF contacts.

You need to find a gas qualified engineer. You’re playing about with your appliance when you’re not qualified to and therefore breaking the law and potentially putting you and any other house occupants at risk, regardless of how confident you might feel. Not what you want to hear admittedly but you need a gas safe registered technician and no one on here will advise you on how to approach this problem.

@ShaunCorbs

SJB060685 said:

You need to find a gas qualified engineer. You’re playing about with your appliance when you’re not qualified to and therefore breaking the law and potentially putting you and any other house occupants at risk, regardless of how confident you might feel. Not what you want to hear admittedly but you need a gas safe registered technician and no one on here will advise you on how to approach this problem.

@ShaunCorbs

Click to expand...

Thank you Shaun for your honest & very useful reply. Horses for courses, I suppose. But given the cost of repairs, I owed it to myself to at least try.