[Tannaton]

Whilst it was a bit direct - Swiss's advice was factual and relevant.

VIDA is by the far best diagnostic system that you can use on your Volvo but it will only tell you symptoms, not causes. Like a doctor know your temperature is high and blood pressure low.... a brain is needed (with experience and intelligence) to diagnose.

By coming on to this forum you get access to more Volvo experience that anywhere else. But as you say you have the option of going elsewhere...

Anyway to your post - I agree with what has been said so far - ECU's very rarely give trouble and generally either work or fail hard.

Bad earth's are not likely to be the issue either - Volvo's wiring standards are much higher now and every resistance based sensor has it's own dedicated earth and reference resistor built in so it is effectively one half of a wheatstone bridge. Many of the sensors now are digital also (but probably not on the Mk 1 90)