I have an annoying problem with my 2001 TS Astra, 1.8L (Im in Aust, & assume its same as your Mk4).
After driving 160000km with the usual routine maintenance, my emission light has come on, and stays on.
When Diagnosed the code shows the code for the oxygen sensor.
Having tried a new sensor and 100% ruled this out as the cause of the problem, I have now been advised that it would either be a ecu or a wiring problem.
One other clue is that from the moment the emission light problem occured. the car has been running up to temp very quickly over a short distance, and the radiator fan always remains running after a short journey.
As for the water temp problem I would normally suspect a thermostat issue. It just seems strange that this seems related to the emission light saga, as its only happened since then.
Q. Would I be right to assume it all relates to a faulty ecu. That is my guess. To complicate matters I cannot easily have the ecu checked before ordering a replacement.
Any ideas appreciated.