Hi Everyone,
Just got back - good to meet you Nick!
Having had a good look over this car today, I wanted to share our findings (and lack of findings) in relation to this, and see if people agree with my line of thinking, or have any other suggestions, based on this update, on where we go next.
The symptoms / complaint have already been described on this thread, so I won't repeat that.
So, first job, was to finish over changing the selector switch. After partially removing the heatshields, I was able to get to the electrical connector for the selector switch, and change it over. The replacement certainly looked clean, and okay.
After installing the switch, I started up the car, and made sure it's aligned correctly. I am sure it is - I say this because the car only starts in P or N, and the relevant lights illuminate next to each gear - without the "multi light" display that's usually attributable to a buggered selector switch.
I noticed the engine light was on (the spanner symbol), so using "Scan My Opel", read the fault codes.
They were:
P1625-02: Internal Power Control Final Stage Low Input.
P0748 -06: Unknown Error
I stopped the engine, and put the ignition switch to "on" but not started (so the dash lights up). I then cleared the codes. The codes momentarily cleared, but, then came straight - and the warning light was back on within seconds. It was the exact same two codes, which returned.
I double checked the selector switch fitment / connections, which were okay, and tried aqain. Codes kept returning.
Given these same codes were also present prior to me arriving, and with the old selector switch fitted - I think we can safely say that sadly this isn't a selector switch problem, but something more complex than first thought. The same codes are returning, regardless of which switch is fitted.
At this point, I decided to check the electricals as thoroughly as one can, given today's rain! I dis-assembled, checked, and re-seated the three large round loom plugs near the battery tray. I also removed, checked, and cleaned the earths. In addition, I visually checked, and removed / reseated the round loom connector, which plugs into the passenger side of the autobox.
What I think is noteworthy, is that the fault codes are persistent, in that they come back immediately after being cleared, even without the engine being started. This, to me, is indicative of an electrical, rather than a mechanical fault. But, I am starting to think the fault is with the electrics inside the box itself, not within the loom / ECU, which is all in good visual condition.
I also tried my code reader - this only came up with one of the codes - the P1625, however, mine described the fault as "TCM Flash Checksum Fault (Saturn)".
I don't for a minute think the gearbox ECU is faulty - I have never, ever seen any ECU fail on an Omega (aside of the ABS ones), and I think the code is more likely a result of something else. I don't subscribe to "the fault code says it's xyz, because we all know the codes can arise from some other input error, somewhere else. But - for completeness, I tried a known good ECU that Nick had spare. Same part number. I installed this, cleared out the codes, and - you guessed it - the same two came back. So, I think on this basis, we can preclude any form of gearbox
ECU problem.