Issue: Engine control unit detects a fault in emissions, ignition, fuel, or sensors. Common on S3: P0420 (catalyst efficiency), P0300 (random misfire), P0171 (lean bank1).

Symptoms: Flashing = severe misfire (stop driving!). Constant light: power loss, rough idle, fuel smell.

🔬 Diagnosis (pro level):

• 1. Scan with VCDS / OBD11 – record all codes AND freeze frame (RPM, load, coolant temp).
• 2. Check short/long term fuel trims; if >±10% suspect vacuum leak or MAF.
• 3. Perform smoke test on intake system (common leak at PCV or intake manifold gaskets).
• 4. Monitor O2 sensor voltages: should switch rapidly (0.1–0.9V). Slow switching = dying sensor.
• 5. Relative compression test via starter current (via diagnostic tool) to find dead cylinder.
• 6. For S3 2.0 TFSI, check high pressure fuel pump (leaking into cam follower).

💰 Costs: €80 (diagnosis) … €180 (O2 sensor) … €650 (injector set) … €2.800 (catalytic converter + labour)

Issue: Critical low pressure – possible causes: oil pump failure, blocked pickup, worn bearings, wrong oil viscosity, faulty sender.

Symptoms: Beeping, red oil can symbol, top end noise, possible engine rattle.

🛠️ Expanded diagnosis: 1) Stop engine immediately – check dipstick. 2) If oil ok, measure actual pressure with mechanical gauge at sender port. 3) At warm idle, pressure must be ≥0.8 bar; if lower, drop oil pan (sludge / broken pickup). 4) Inspect oil filter housing and cooler seals (leaks). 5) For S3, check if oil pressure switch connector is brittle (common).

🔧 Repair cost: €50 (oil change & switch) … €1.200 (oil pump & pickup) … €3.500 (engine bearings if damaged)

Issue: Overheating or low coolant. Known S3 issues: water pump (plastic impeller fails), thermostat stuck, coolant temp sensor G62.

Diagnosis deep: 1) Check expansion tank level (cold). 2) Pressure test cooling system (leaks). 3) Monitor live data: actual coolant temp vs. ambient; if temp jumps erratically, replace sensor. 4) Check for white smoke (head gasket). 5) Infrared thermometer on radiator hoses – if lower hose cold while upper hot, thermostat stuck.

💰 Cost: €15 (coolant) … €230 (water pump 8V) … €480 (thermostat + pump + labour)

ABS / ESC light: Wheel speed sensor, ABS pump, or steering angle sensor. Symptoms: ABS inactive, speedo dead, stability control off.

Advanced diag: Read fault codes; check resistance at sensor (1.2–1.8 kΩ). Check reluctor rings for cracks. Live data: compare wheel speeds while driving – deviation indicates faulty sensor.

➕ Brake pad wear indicator (BRAKE light): On S3, when pads reach wear sensor, dash shows “BRAKE PAD”. Check pad thickness, replace sensor if triggered. Cost: €180 (front pads + sensor)

💰 ABS cost: €70–€150 per sensor, €400–€800 for ABS pump module

Issue: Fault in supplemental restraint system – could be seat occupancy sensor, belt tensioner, crash sensor, or clock spring.

Symptoms: Airbag light stays on, passenger airbag off light may blink.

Diagnosis: VCDS / dealer tool required (generic OBD2 often doesn’t read SRS). Check under seats for disconnected yellow connectors (corrosion). Clock spring (steering wheel) – test continuity with multimeter.

Cost: €120–€200 (reset & diag) … €350 (clock spring) … €650 (occupancy mat)

Steering warning (yellow/red): Power steering failure (electric on S3 8V/8Y) – could be rack, motor, or torque sensor. Diagnosis: check battery voltage (low voltage triggers fault), scan for fault codes, perform calibration with VCDS.

Electronic parking brake (P) light: Stuck caliper, switch fault, or control module. Use VCDS to run output tests on caliper motors. Cost: €250 (rear caliper) … €150 (switch)

• TPMS (tyre pressure): indirect or direct. Diagnosis: check pressures, reset; if returns, inspect for puncture. Use sensor tool to trigger each sensor. Cost: €0 (air) – €320 (sensors)
• Washer fluid warning: low fluid or faulty sensor. Check reservoir and wiring. Cost: €5 top-up / €40 sensor.
• Battery / alternator: voltage below 12V or alternator not charging. Diag: measure alternator output; check belt, voltage regulator. Cost: €180–€800