I'll give you a few ideas as to how you could have problems with ECU repairs.
a) the circuit boards are interchangeable, so they could buy second hand units and swap out your old unit, but not charge for the second hand unit, but it is called a "testing fee" and a "repair fee", stuff you would never know, unless you are an electronic engineer.
b) Take up so much time that you want the unit back and you are willing to pay the "testing fee" for having had the unit sit on their desk for 2 months and carriage.
c) New unit needs coding to old unit - another charge of £50 or more for doing nothing that some garages can do by hand-held computer. They might even suggest a new chip is required to do it, when the unit is uncoded - depends on the year and make.
Look at what they say they will do - how they test stuff, then read their competitors website and compare the wording. What do their Ts & Cs say? Bigger companies and more reputable companies have more Ts & Cs.
If they charge more than £40 or so for testing, either they are incompetent or trying it on. Any electrical engineer would be able to diagnose an ECU fault within an hour or so. There's only about 6 chips and a few tens of transistors, resistors, capacitors and diodes. Once you have basic data about the failure, some parts of the ECU are obviously working OK, so you narrow your testing to the specific areas affected. Experience helps a lot.
