My 92 turbo legacy has no signal from the ECU to the O2 sensor, and nothing coming from the O2 sensor either. I put a Japanese engine in about 2 years ago. Could the ECU/O2 sensor have been 'switched off' to make it run richer or something? Everything else seems to work electrically. Should I be checking for a signal right at the ECU, not the connector for the sensor? The car has a hesitation in driving sometimes which I was starting to think could be the knock sensor, based on other posts. Which sensors are most important for the car to drive well- presumably the MAF is most important, then the TPS? I once had a check engine code for the O2 sensor some months ago but it never came up again after clearing the code. I have also had a couple of knock sensor codes stored but these don't come up again after clearing. If the knock sensor was faulty should it come up with a code immediately? Also, if I disconnect the O2 sensor no code comes up. Should it? When the car is behaving, it runs really well. When it gets the problem it lacks power, hesitates on accelerating, and appears to 'miss'. Any ideas? Thanks...

Since you state that the ECU does not give a code when the O2 sensor input is removed I would think the input has been bypassed somehow. I could be that it may take some time for the code to be generated but I would think that pulling the input would send an immediate alert to the ECU.

 

As far as the intermitant hesitation goes I would tend to think there may be an ignition or fuel issue causing this. Replacing the knock sensor may help with this but I would check the ignition first.