🔍 Issue: ECU detects misfire, emissions, sensor drift, or catalyst issue. On A6 common for P0299 (turbo underboost), P0420 (catalyst), P0300 random misfire.

• Rough idle / shaking at stoplights
• Fuel trim deviations (rich/lean)
• Reduced power / limp mode
• Failed emission test

Diagnostic fee at 24Car-Repair: $135 (includes full VAG scan).

• Red light with buzzer / “Oil pressure low” text
• Top end noise (lifters, timing chain)
• High oil temperature possible

• Red “( )” light or “BRAKE” message
• Pedal goes too far / spongy
• Squealing due to wear sensor
• Low fluid level (mineral or DOT4)

• ABS inactive, but brakes work
• Light stays on after driving
• “Stability control fault” message

🔧 Diagnosis: VCDS scan for wheel speed sensors (G44–G47). Check reluctor rings (toothed wheels) for damage. Measure sensor resistance (1.1–1.8 kΩ). Inspect wiring at suspension struts. Test ABS pump operation via output tests. For A6 C7, common ABS module failure (rebuild possible).

• Airbag light stays on (passenger off LED may flash)
• Passenger occupant detection error

📋 Diagnosis: Use VCDS (cannot be done with generic OBD2). Check under‑seat connectors (yellow) for corrosion. Inspect clock spring (horn, steering wheel buttons). Test seat belt tensioners. For passenger occupancy mat (common C7), measure resistance. Clear codes after repair.

• Headlights dim / flicker
• “Alternator” or battery symbol
• Start stop unavailable

⚡ Diagnosis: Measure battery voltage (12.5V+ engine off). Running: 13.8–14.8V. Check belt tension (poly‑rib). Use VCDS to check generator load signal. Test battery health (carbon pile). For LIN alternator, check wiring to regulator.

• Red temp light / needle in red zone
• Steam, coolant smell
• “Stop! Check coolant” message

🌡️ Diagnosis: Check expansion tank level (cold). Pressure test cooling system (leaks). Scan for temp sensor plausibility (G62). Check fan operation (A/C on, VCDS output test). For 3.0T, water pump weep hole leak common. Verify thermostat opens (temp rise at highway).

• Yellow (!) symbol
• “Tyre pressure loss” warning

🔎 Diagnosis: Inflate to placard. Use TPMS tool (e.g. Autel) to trigger sensors – check battery (5-8 years). Relearn via MMI (store pressures) or VCDS. For indirect systems (ABS based) check wheel speed sensors.

• “AdBlue low” / countdown starts
• No start after counter reaches zero

🧪 Diagnosis: Check AdBlue level in tank (via MMI). Use VCDS to read reductant quality, injector quantity. Inspect DEF tank heater (winter failure). Check NOx sensors (P20EE).

• Engine runs rough (cold start)
• Excessive smoke on startup

🔌 Diagnosis: Scan for glow plug circuit codes. Measure resistance of each plug (0.5–2 ohms). Check glow plug relay (J179) control signal. For 3.0 TDI, also check pressure regulating valve.

• Red steering wheel symbol
• Heavy steering, assist reduced

🔧 Diagnosis: VCDS check steering angle sensor (G85) calibration. Test torque sensor in column. Measure power supply to steering rack. In C7, common rack failure (moisture).

• Flashing gear indicator / “Gearbox malfunction”
• Limp mode, no reverse or certain gears

⚙️ Diagnosis: Scan mechatronic unit (VAG). Check fluid level and condition (DL501/382). Read clutch adaptation values. Perform basic setting after repair. Common mechatronic pressure sensor failure.

• Orange soot symbol
• Regeneration required / passive regen

📈 Diagnosis: Measure differential pressure sensor, soot mass via VCDS (>45g = clogged). Forced regeneration (must meet conditions). Check oil dilution.

• “Pre sense fault” message
• ACC not available, camera blocked

📡 Diagnosis: Check radar sensor alignment (bracket), clean camera area. Scan for calibration errors. Static calibration required after bumper removal.

• Car sitting low / one corner down
• Compressor runs continuously

🔧 Diagnosis: Check compressor relay, piston ring (rebuild kit). Inspect bags for cracks (soapy water). VCDS read level sensor (G76, etc.).

• Yellow light with arrows
• Static cornering light fault

💡 Diagnosis: Check level sensors on control arms (broken link). Scan for G76/G78 faults.

• Yellow lane symbol / fault
• Camera view limited

📷 Diagnosis: Clean windshield area near rearview mirror. Calibration required after glass replacement.

• Red (P) flashing
• Button not releasing

🔩 Diagnosis: Check switch, rear caliper motors (VCDS basic setting). Common stuck motor (C7).

• Blue symbol / “Washer fluid low”

💧 Fix: Top up – if light remains, check sensor float (common stuck).

• Red car outline with open door

🔧 Diagnosis: Check latch switch (bonnet double lock). Microswitch in door lock. VCDS measuring blocks.

• Green A with slash / message

⚡ Diagnosis: Scan for battery management codes. Check brake vacuum sensor, hood switch.

• Yellow triangle with exclamation – general message

📋 Diagnosis: Check vehicle status in MMI. Often combined with other warnings. Scan all modules.