1. When I got the car, it was running badly. Cylinder 1 only had about 4 bar compression due to a leaking valve. At the time I thought I had found the issue, but that was only part of the problem. After that I replaced the ECU, since the one fitted was from a 4.0 L engine and mine is a 3.2 L.
2. The other sensor seems to behave normally like an oxygen sensor, switching around roughly 1–5 V. It is only the signal voltage that is stuck at 4 V, both sensors have 12 V.
3. The car is drivable, but it doesn’t run nicely. The gearbox struggles to decide which gear to pick and shifts quite harshly. I recently did an emissions test and there is a lot of unburnt fuel in the exhaust. Interestingly, under full throttle the car pulls fine and the gear shifts also feel normal, which makes it a bit confusing since there is no module for the gearbox on this model.
4. I will provide more info next week since the workshop is closed for holidays right now. Basically, the good signal looks like the picture below, while the other one is just stuck around 4 V. Keep in mind that these oxygen sensors range from 0-5 V instead of 0-1 V like the newer ones.

 

OK.

As you have a Picoscope, it would also be useful to run a Cylinder Balance Test when the engine is running badly. Lots of info online on how to do that. That would help identify if there is a problem with a full bank of cylinders or just one or two in the bank.