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.
1991 Audi 100 BaseFACTORY OEM SERVICE PROCEDURE
OXS Frequency Valve
- Remove harness connector from OXS frequency valve. Connect LED Test Light (US 1115) Red lead to terminal No. 1 (Blue/Black wire). Connect Black lead to terminal No. 2 (Red wire).
- Perform steps in OUTPUT CODES (for 2.2L) under RETRIEVING CODES in TESTS W/CODES article in the ENGINE PERFORMANCE Section. When code appears, LED test light must be off. Replace OXS frequency valve if light is off but OXS frequency valve does not work.
- If light comes on, ensure battery voltage exists between OXS frequency valve terminal No. 1 (Blue/Black wire) and ground. Repair wiring or circuits if necessary.
- If battery voltage exists, turn ignition off. Disconnect ECU harness connector. Ensure resistance in wire between ECU terminal No. 8 and OXS frequency valve terminal No. 2 (Red wire) is 10 ohms. See Figure for ECU terminal identifications. If resistance is not as specified, check for open circuit in wire. If wire is okay, replace ECU.
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.