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.
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HOMESERVICE MANUALSAUDI199590 QUATTRO SPORT V6-2.8L (AAH)REPAIR AND DIAGNOSISPOWERTRAIN MANAGEMENTCOMPUTERS AND CONTROL SYSTEMSDESCRIPTION AND OPERATIONON-BOARD DIAGNOSTICS OVERVIEW
1995 Audi 90 Quattro Sport V6-2.8L (AAH)
On-Board Diagnostics Overview
1995 Audi 90 Quattro Sport V6-2.8L (AAH)SECTION On-Board Diagnostics Overview
ON-BOARD DIAGNOSTICS (OBD)
The ECM monitors sensor inputs and its own output signals, comparing these values with those stored in its memory. If a signal deviates greatly from what the ECM "knows" is the correct value, a diagnostic trouble code is stored in the fault memory. The ECM may substitute a good signal value from its memory to provide good driveability. Because of this substitution of signals, the operator may not be aware that there is a problem. If the fault is emission related, the ECM will store the fault code until the memory is erased. For non-emission related faults, the memory is cleared each time the engine is started, so if a fault is suspected, codes should be displayed and recorded after the fault has been observed and before the engine is re-started.
The ECM monitors sensor inputs and its own output signals, comparing these values with those stored in its memory. If a signal deviates greatly from what the ECM "knows" is the correct value, a diagnostic trouble code is stored in the fault memory. The ECM may substitute a good signal value from its memory to provide good driveability. Because of this substitution of signals, the operator may not be aware that there is a problem. If the fault is emission related, the ECM will store the fault code until the memory is erased. For non-emission related faults, the memory is cleared each time the engine is started, so if a fault is suspected, codes should be displayed and recorded after the fault has been observed and before the engine is re-started.
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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.