"Your ecu is fried sir, yes I believe it was on it's last legs and gave up while I was testing it."
I once got exactly that from a main dealer!
In the early days of ECUs,before each was "personalised" to the individual vehicle, mechanics would buy a bunch of ECUs from the wreckers.
If some strange fault occurred, they would always say "it's probably the computer" & slot one of their spares in.
Our 1988 Australian Ford Falcon would gracefully come to a halt & as groping around in cars had lost its appeal, I called the "mobile mechanic" who chucked a spare ECU in & called it good.
The Ford strenuously disagreed, & finally gave up entirely, so I was stuck with borrowing an oscilloscope from work (a THS7xxx---can't remember the exact model) & checking the thing for myself.
Back probing the weird connector on the ECU, I found that all the signals were present, including most importantly, the ignition pulses, which were then fed to the old style distributor, but seemingly, didn't arrive there.
It turned out to be an old fashioned car type intermittent, where the wires in the plug which went into the distributor had fractured, making intermittent contact.
Digging round in the junk box, I found some bits that I could use for a bodge fix, & the old beast was a "go-er" again.
The Falcon had another, very intermittent fault, in that every now & then, it would hesitate when cruising at speed, then "come good".
This, over years, got a bit more annoying, so I revisited the ECU, this time using my old BWD which had a triggering fault.
All the pulses were there, sliding across the screen.
It turned out to be an incorrectly fitted injector in the throttle body, so again, nothing to do with the ECU.
OK, car electrics have moved on since then, but basic stuff like connectors still live in a hostile environment, so the old "trace the circuit" skills are still important.