I have a Trane XL80 that is around 10 years old. There is an intermittent problem where the furnace burners sometimes fail to ignite. Sometimes when they do ignite they go out before they should. The engineer came and fitted a new flame sensor, but this made no difference. He then claimed the problem was a faulty electricity supply (ground fault?), based on the controller's LED indicator flashing twice. However, I was not persuaded by this as there was no other evidence of such a problem, and the furnace had been working fine for several years. As an experiment I temporarily re-wired the furnace to a new dedicated circuit with its own brand new breaker in my house circuit breaker box, and as I expected the problematic behaviour persisted.
A couple of years ago the same engineer fitted a new controller after the heating system failed. I wasn't around at the time, so I don't know if this is related or not.
My hunch is that the gas valve is somehow sticking shut.
I have noticed that although the problem is intermittent, whenever it does occur, if I tap the gas valve moderately firmly then it suddenly corrects itself and the burners light. (I listen for a click which is presumably some sort of relay feeding current to the valve, and if the gas doesn't light immediately, I give the valve a tap right away.) I believe the electricity required to open the valve is 24 volts AC, and as far as I can tell, this supply is fine.
Can anybody advise what is going on? I am fairly sure a replacement gas valve will fix it, but my engineer does not agree.

If you are taping the valve and it comes on odds are it is most likely the gas valve. I have never seen a flame sensor go bad, they either work or need cleaned. I say change your gas valve and leave your engineer at home where he belongs

I think I just fixed it! The cause was a loose wire in the electrical plug connected to the top of the valve. Tapping the valve turns out to be a sledge-hammer-to-crack-a-nut way of jiggling the wires in the connector. When I tugged on the wire coming from the pressure valve into the gas valve, I was able to make the gas valve open and close at will (while the igniter was glowing).
I just needed to tighten the connection.
Hope I didn't damage the valve by hitting it...
Thanks for your advice, Joshdta. Would anyone like to comment on whether my engineer should have detected this fault or not? It would help to know if he's worth hiring in the future.

It is hard to find things that work sometimes and sometimes not. But should have been found. Like I said changing the flame sensor could not have been the probem

AlistairImrie, have you had any problems since you fixed it? I had exactly the same problem since last Saturday. Touching the 2 wires going into the volve helped lit from time to time, but not always.

No problems so far, although it hasn't had to fire up many times. But it has fired up enough times to give me great confidence I've fixed it. I tugged the wires reasonably hard (just with my fingers) to verify I'd found the cause. (If the wires come out then you can be sure they weren't in securely enough!)
I tightened the connection by forcing a tiny screwdriver blade down the split of the pin in the valve's connector. This widened it a little, enabling a firmer grasp of the pin it connects to.