I absolutely "get" what you are saying about circuit boards PB. The main reason why we had a new boiler when the kitchen was done -10/12 years ago now - was because the circuit board in the old one failed during the makeover and the price of repair was "silly"
Our boiler was fitted in November 1995, and is still working well.
It had to have the inner flue tube replaced due to corrosion. This had upset teh pressure sensors, so the associated relay block had to be replaced too. Just over £100, done by a local plumber. That was a few years back now.
Then more recently the fan motor seized. Got one off Ebay, fitted it myself in an hour or so. It is only the gas stuff we should not touch, the mechanical and plumbing stuff is fine. Nothing as complex as under the bonnet of a car these days.
Sadly, the seizing motor had upset the relays on the circuit board, so it would then start up, run the fan for a few seconds, then fire up the flames, and immediately turn it all off again. Not much heat to the water then.
An internet search showed it as a common problem, a result of the seizing motor.
Choices:
1. Send circuit board away for repair, turnaround 5 days. Cost £60
2. Order reconditioned board, pay surcharge, refundable on return of old one. Delivery, 2-3 days. Cost the same £60 after surcharge refunded.
3. Drive 8 miles to the plumbase distribution centre (just luckily local) and get a brand new genuine potterton one. £148.
Weather cold, near freezing actually, no other heating apart from one fan heater and an electric fire.
So we paid the money, went for a drive. 45 minutes round trip. Fitted and working within 20 minutes of arriving home.
Washing machine circuit boards though - no diagnosis structure, no easy source for replacements, scrap the machine.
Got hold of the car and "threw it" at the ramp stops at one end. It bounced back from them to the rear stops and then came to rest above us. "Now, just by eye, see if the smaller of the two gaps is roughly the same as the guage" if it is, you're done. If not, adjust it and remember to "walk" the car back and forward each time you do so the belts can resettle.
We had a pit. Always did belts over the pit, always rolled the car back and forth to settle the belts.
Did my own 66 adjustment. Rear end on stands, just roll the wheels to settle the belts. Have to roll both wheels together, so lying under the car, arms outstretched, easy to pull muscles. Luckily don't have any!