It does happen, especially when some over-complicated brainbox goes mental. But a decent garage should bill you nothing for failing to diagnose a fault.
This is why I prefer to diagnose everything myself and only pay a spanner monkey to do the mucky stuff.
Ask them if they have the manufacturers approved software for fault finding. If the answer is no, ask them wtf they took the motor in for in the first place!
these days I just want a simple car which just works and won't present me with a wallet busting bill, does such a car exist? Tempted with electric but prices are just crazy here once taxes are added on. 2nd hand? don't fancy an £8000 bill when the battery needs to be replaced ? Some of the Chinese electric cars however are looking great, interesting times.