This car failed its MOT emissions :high CO 0.7%, Lambda < 0.97, HC passed, fast idle test failure. The car passes its emissions on the idle test.

It looks like the cat is dead as this is a high mileage vehicle. I also found a massive air leak to the crankcase breather system using my smoke tester . The repair with new pipes resealed the crankcase breather system with no more smoke leaks, however this did not cure the CO problem.

I then was looking at the O2 sensor (pre-cat only) and found that the switching voltage although approx 1Hz at fast idle was fixed between 0.4 volts and 1.2 volts on my Pico.

The sensor was 4 wire type with heater wires which tested OK.

The output from the ECU to the signal wire was fixed at 0.3 volts with KOEO.

Is this normal?. It appears that this O2 sensor is working correctly but switching with a 0.3V offset?

My understanding and training only ever talks of correct switching being between 0.1 and 0.9 Volts for a 1 volt sensor.

Please advise , as I think this O2 sensor is working OK and the ECU is operating with a fixed offset, but I have not seen this before.