'94 945 Turbo. B230FT w/ E.G.R.

Can an O2 sensor be bad or poorly performing and NOT cause a fault code or Check Engine light in the OBD I system?

Still trying to track down cause of high 1800 RPM idle after W.O.T. driving.

T.P.S., E.C.T., I.C.V. and E.C.U. all replaced with known good or new parts and condition still exists. Throttle body and T.P.S. is spotlessly clean and adjusted. No vacuum leaks whatsoever. Injectors removed, cleaned and resealed. New fuel filter.

All new secondary ignition parts. Runs great, just high idles after W.O.T.

If I shut car down and immediately re-start, idle is normal

Yes they can. Have you unplugged it while its high idling?

Is the high idle present with the AMM disconnected? Does disconnecting it stop the high idle?

An O2 sensor can be worn and not throw a code (they get slow as gunk accumulates in them, I think), but you'd see more minor problems.

Which ECT did you replace? There are 2 or 3. How's the IAC? Run the OBD1 self-test and see if it thunks like it should, or if it hangs up. Also, removing it and checking it by hand (One user replaces it with a gas-valve, I forget the diameter) and for operation.
The other easy check would be unplugging it while the car is running (it should stall), but I'm not sure that's safe for the ECU.


If you shut down the car after a high idle, are the plugs soaked in fuel?

anythings possible.

with that said a bad o2 sensor is more likely to cause very poor running conditions than merely a high idle.

you can easily check it for spec operation using a multimeter set to 2 volts dc and seeing how the reading swings back forth between .1 and .9 volts.

rapid cycling is what you are looking for.

i would if i were you check my egr system for clogs