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
Oxygen (O2) Sensor
- Disconnect differential pressure regulator connector from side of regulator. Using Adapter (1315 A/1), connect Multimeter (US 1119) to differential pressure regulator and to the connector removed from regulator.
- Set multimeter to 200-mA DC scale. Disconnect O2 sensor connector. Start engine and let idle. Note reading on multimeter. Ground male end of O2 sensor connector.
- Observe multimeter. Current should increase from 9 mA to 11 mA after 15 seconds. If multimeter reading is not as specified, check wiring between FICU and O2 sensor harness connector.
- If wiring is okay, replace FICU. Connect O2 sensor and start engine. Raise engine speed to about 3000 RPM for a few seconds.
- Differential pressure regulator current must return to original mA value, and fluctuate approximately 1-3 mA in the -10 10 mA range. If current does not return to original mA value or does not fluctuate, replace O2 sensor.
Factory Service Tools
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Disclosure: Verified tool recommendations may earn an affiliate commission at no cost to you.
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.