Pre-Cat Oxygen Sensor, Bosch - E46 M3, MZ3 S54 (11781743994)

E46 M3, MZ3 S54 bank 1 pre-cat O2 sensor (cylinder 1-3, before the catalytic converter).

The pre-cat O2 sensors, also known as Lambda probes, on the E46 M3 are some of the most important sensors on the S54 engine. The pre-cat sensors tell the engine computer how well the engine is working (note: not how clean it is, that's the post-cat sensor). The pre-cat sensors give the ECU feedback on the engine tuning - if it's running lean or rich - and the ECU makes adjustments based on their data. Each bank of cylinders has its own sensor before and after the catalytic converters. The ECU adjusts fuel injection based on the data so the sensors also affect fuel economy in addition to performance. An integrated heater circuit helps prolong the life of the sensor but can also fail and give a check engine light.

Signs of an O2 sensor failure include worse fuel economy, lack of torque, and fault codes for the sensor(s). It is a good idea to replace the O2 sensors at regular intervals rather than waiting for a failure. Since their performance degrades over time you may not even notice the loss in performance or economy. Since the complete O2 sensor is part of the emissions system, a bad sensor may not set multiple OBD Readiness flags.

This is a genuine Bosch E46 M3 O2 sensor. Bosch is the original supplier to BMW for oxygen sensors, as well as the inventors of the system.