BMW E34 oxygen sensor fault: testing and replacement
How is the E34 oxygen sensor checked?
The oxygen sensor generates its own voltage, so it is measured as voltage, not as resistance. With a warm engine at idle, the voltage on the signal wire varies between about 0.2 and 0.8 V, about once a second. A low reading alone does not prove the sensor is faulty.
How is the oxygen sensor heater checked?
The engine management can set a fault code for the heater, and a code can appear even when the heater works. Check the heater relay and its control from the pin on the engine control unit. The relay must not be given constant power, as that would drain the battery.
Do other faults need to be sorted out before replacing the oxygen sensor?
Yes. Misfires let unburnt fuel into the exhaust, and the engine management may interpret that as a sensor fault. Fix the ignition and other leaks first. Oil leaked from the valve cover gasket into the spark plug wells can cause misfires. Adjust the mixture after fixing the leaks.
Where is the oxygen sensor and how is it replaced?
The sensor is in the exhaust pipe. On the M30 engine the replacement needs a 22 mm wrench: unplug the connector, unscrew the old sensor, screw the new one in tightly and plug the connector back in. The M60 has two heated sensors, and the driver's side sensor covers cylinders 5-8.