How many O2 sensors does a wrx have? I have been told that the earlier Versions have two one before the turbo ie. manifold near the oil filter and one after the turbo ie downpipe. A comparison of the two readings by the ecu is made to determine the correct air fuel mixture. Later versions only have one O2 sensor ie after the turbo.

Can anyone confirm this is true? Are the readings of the O2 sensors sampled by the ecu continuously throughout the rev range.

Ther is only one oxygen sensor, It's in the headers pre turbo) on earlier cars, and in the down pipe (post turbo) on later cars.

O2 readings are only used when car is on part/light throttle.

Exhaust lambda sensor's

The lambda sensor is in the off-side header on early WRX's. The other sensor in the downpipe is a temperature sensor for the catalytic convertor .On my WRX I have been able to unplug the temperature sensor without any engine check lights illuminating.For information I can tell you the mid-section on my WRX was only a resonator.

regards,
John

But what do I know....