I have a '94 325i that is giving Code 1221, O2 Sensor Error. There is only 1
(one) O2 Sensor.
The troubleshooting guide states to connect my meter and look for 0.2 to
0.8vdc at idle. I have 0.45n right after engine start, and it drifts to
0.86n (where the value of n floats in a narrow range). It takes a short
delay for the voltage to change from .45
If I increase the engine speed slowly, the reading remains at .86n up to
2500 rpm -- I don't go above 2500 for no particular reason, but the car is
on jack stands in my garage so that's a good a reason as any. This test
tells me that the air/fuel mixture remains constant on slow changes.
If I jump on the gas quickly then release, the voltage drops to the 0.2nn
range for a moment, then comes back to 0.86n. This test tells me that the
air/fuel mixture goes rich when the throttle is opened suddenly, then goes
to a normal range as the airflow catches up. I got a few readings of 0.19n,
and I can repeat these readings anytime I want by rapidly pressing and
releasing the gas pedal.
My troubleshooting guide does not say if a high reading is rich or lean, and
maybe I have the figures reversed. Perhaps my car is rich at 0.8, and the
rapid press and release causes 0.2 and this is lean because the air changes
faster than the gas. I expected 0.8 to be lean, and 0.2 to be rich because
pressing the gas pedal would cause lots of gas to be delivered before the
air got there.
I have 12v on the heater and 4.5ohms across the heater.
According to the troubleshooting guide (ROBERT BENTLEY PUBLISHERS for my
car), I have a good sensor. The Check Engine Light with Code 1221 persists.
The only fault I see is that the sensor is expected to read 0.8v, and I have
0.9 (rounded). What am I missing?
The car runs fine.