Niannorth,
It sounds like a PCB fault. I have a similar boiler which died just before Xmas with the same failure mode you’ve described. I could also see the spark now and then after the flame was lit. I also noticed gas modulation was unstable. Red light would come on a few minutes after start-up. I replaced the PCB. Boiler works fine now.
Replacing the PCB is a rip-off, I agree. MTSgroup (Ariston is part of Merloni TermoSanitari) don’t offer any repair service. The fault is probably just a resistor/capacitor/diode etc just out of spec in the flame detection circuit and scrapping the whole PCB for the failure of a penny component is eco-unfriendly. BUT it needs electronic skill and a test kit to fix this type of fault properly. Flame detection is VERY important.
MTS make a range of boilers, sold throughout Europe, all using the same technology. As far as I can tell, they use a Flame Ionization Detector (FID) rod to establish whether the burner has been lit properly by a spark generated at two similar rods next to it. A FID, is analogous to a cathode in a TV tube, when heated it gives off a stream of electrons. In a boiler, these charged particles flow into the burning gas. It’s this current flow which is detected by the PCB circuitry. It looks as though you were measuring the current change OK and the PCB was seeing the current drawn. The PCB is just not throwing the “detected switch” properly.
Generally, in this type of boiler technology, you can rule out the probe as the likely fault. Provided there’s no obvious resistance measured from the end of the probe to the PCB connector.
(From this thread, you’d think a flame detection problem is rocket science!)
I’m still looking for somebody who can properly fix my old board. (I’ll keep it as a spare.) CET (I assume this is where you saw the reconditioned offer) told me they’re a bit busy this month but they’ll look at mine later. Any other suggested repairers would be an useful post?