Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
1990 BMW 535iFACTORY OEM SERVICE PROCEDURE
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.
Factory Service Tools
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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