Pre-Cat Oxygen Sensor, Bosch - E34, E24, E32 (See List) - 11781720537

E34 535i, E34 M5 (US), E24 635CSi (88-89), E32 735i/735iL pre-cat O2 sensor (located before the catalytic converter).

The pre-cat O2 sensor, also known as a Lambda probe, on the 535/635/735 is one of the most important sensors on the M30 engine. The pre-cat sensor tells the engine computer how well the engine is working (note: not how clean the emissions are). The pre-cat sensor gives the ECU feedback on the engine tuning - if it's running lean or rich - and the ECU makes adjustments based on its data. The ECU adjusts fuel injection based on the data so the sensor also affects 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.

Please note these fitment details: 

On 635Csi - For 1988 and 1989 models ONLY. 
On 535i - For 1989-1993 models. 
On M5 - For US 1991-1995 models. 
On 735i / 735iL - For 1988-1994 models. 

Signs of an O2 sensor failure include worse fuel economy, lack of torque, hard starting, and fault codes for the sensor(s). It is a good idea to replace the O2 sensor at regular intervals rather than waiting for a failure. Since the performance degrades over time you may not even notice the loss in performance or economy.

This is a genuine Bosch O2 sensor, with Bosch being the original supplier to BMW for oxygen sensors, as well as the original inventor of the O2 sensor. You can find no higher quality replacement O2 sensor for your later model M30 big six powered BMW. 

BMW Fitments:

5 Series
E34 (89-95) > 535i
E34 (89-95) > M5 (91-95)
6 Series
E24 (77-89) > 635CSi
7 Series
E32 (88-94) > 735i
E32 (88-94) > 735iL