If you want to check the pins on your ECU, and you happen to drive a later model car, it is by the passengers feet (assuming this is a LHD car in the US). Water does get down there on some model years, and has been known to corrode the pins over time. When you unplug the two connectors, check for green corrosion stains. Even if you see none, make sure to give it a spray down with some electrical contact cleaner spray before plugging it back in.
If you have an older XJS, the ECU may be elsewhere.
Please make sure to add the details on your car in a signature so you don’t have to do it again.