O2 Sensor Heater Relay
- On 325 Series, O2 sensor heater relay is on bracket on left front shock tower (relay closest to power steering reservoir). On 525i, O2 sensor heater relay is in relay box in right rear corner of engine compartment (relay closest to outside of vehicle).
- Note wire colors and relay terminal identification. Apply battery voltage to both Green/White wire terminals of relay. Ground the Brown/Green wire terminal of relay. Voltage should now exist at Green/Blue wire terminal of relay. Replace relay if no voltage.