At Audi, code P1391 (or P1391 00) indicates a critical discordance between the signals from the crankshaft sensor (G28) and the camshaft sensor (G40). This fault disrupts the management of timing, injection, and ignition, particularly on TFSI/TDI engines equipped with VVT (Variable Valve Timing) systems.
Common Causes on Audi
- Mechanical Problems (50% of cases):
- Slack timing chain (faulty hydraulic tensioner on 2.0 TFSI engines, e.g., EA888).
- VVT sprocket misalignment (carbon deposits locking the variator).
- Damaged crankshaft tone wheel (cracks on dual-mass flywheel).
- Electrical Failures (30%):
- Faulty G28 or G40 sensor (often after engine disassembly).
- Chewed/corroded wiring (especially near the engine block or rear harness).
- Engine ground issue (corrosion at the anchor point near the battery).
- Other (20%):
- Degraded engine oil (unsuitable viscosity or exceeded oil change interval).
- Weak oil pump (insufficient pressure for VVT).
- Faulty engine control unit (ECU) (rare cases, requires advanced diagnostics).
Specific Audi Symptoms
- 🚨 Check Engine Light + ESP Light on (stability control deactivated).
- 🔧 Misfires at idle (especially on Audi A4/A5/Q5 with 2.0 TFSI engine).
- ⚠️ Sudden stalling during acceleration.
- 🔊 Metallic rattling when cold (slack timing chain).
- ⛽ Excessive fuel consumption (+15-20%).
Audi-Specific Diagnostics
- Required Tools:
- VCDS (VAG-COM) or ODIS scanner to read measurement blocks (groups 090-093).
- 4-channel oscilloscope (to compare G28/G40 signals).
- Key Parameters to Check:
- Phase deviation TDC/BDC (target: < ±3°).
- Oil pressure (minimum 2.0 bar at idle).
- VVT angle (compare with factory values).
- Priority Tests:
- Chain inspection: Check play with a timing tool (e.g., T40058 tool for EA888).
- Electrical check:
- G28/G40 sensor voltage: 5V (supply) / 0.1–4.8V (signal).
- Resistance: 500–1000 Ω (out of tolerance = faulty).
- Oil analysis: Presence of metal particles (indicates pump or chain wear).
Common Repairs & Costs
| Cause | Solution | Average Cost (Audi) |
|---|---|---|
| Failed chain tensioner | Replace chain + tensioner + guides | 900–1,500 € |
| Faulty G28 sensor | Replacement (Ref. 06H906433B) | 120–200 € |
| Stuck VVT sprocket | Ultrasonic cleaning or replacement (Ref. 06H109257) | 300–600 € |
| Damaged tone wheel | Flywheel replacement | 800–1,400 € |
| Wiring issue | Harness repair + heat sleeve | 150–400 € |
⚠️ Warning: On EA888 Gen 2/3 engines (Audi A3/TT), a P1391 can hide premature chain tensioner wear → Risk of engine failure!
Audi Case Studies
- Audi A4 B8 (2.0 TFSI):
- Code P1391 + cold start rattling noise.
- Cause: Failed chain tensioner → Timing kit replacement.
- Audi Q5 (3.0 TDI):
- P1391 after timing belt replacement.
- Cause: Incorrect BDC sprocket timing → Realignment with T10340 tool.
- Audi TT RS (EA855):
- P1391 under acceleration.
- Cause: G28 sensor flooded with oil (faulty seal) → Replacement + cleaning.
Post-Repair Procedure
- Reset adaptations:
- Via VCDS: [Select engine] → [Adaptations] → Reset “Cam Angle”.
- Idle learning:
- Let run for 10 min at 2,500 RPM, then stabilized idle.
- Road test:
- Verify no code over 3 drive cycles.
Audi FAQ
Q1: Can I drive my Audi with a P1391?
→ Dangerous! Risk of engine failure (especially on TFSI) → Towing recommended.
Q2: Why does the code return after replacing sensors?
→ Imperatively check ECU supply voltage and mechanical timing.
Q3: Can a general mechanic repair it?
→ Only an Audi/VAG specialist masters the specific timing tools (e.g., T40267).
Conclusion
P1391 on Audi often signals critical timing wear or a complex electrical fault. Recent TFSI/TDI engines are particularly vulnerable.
Recommendations:
- Stop the vehicle immediately.
- Have the chain tension checked (timing tool mandatory).
- Prefer OEM parts (generic parts cause recurrences).