Common VW & Audi fault codes and what they mean

The twelve fault codes VW and Audi owners search for most — misfires, lean mixtures, boost problems, cam timing and catalysts — explained in plain English, with the real-world causes and how to read and clear each one.

Updated 17 August 2026 · 8 min read

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How to read a VAG fault code

Most of the codes below are generic OBD-II (SAE) powertrain codes — the ones that start with P0. The letter P means powertrain, and the number that follows describes a specific condition the ECU measured. VW and Audi also store their own five-digit numbers (for example 16684) in the older KWP format; a scan tool usually shows both the P-code and the manufacturer number for the same fault.

The key thing to understand is that a code names the symptom the ECU saw, not the part that failed. "System too lean" tells you the mixture was wrong; it does not tell you whether that was a split hose, a tired sensor or a failing fuel pump. Use the code to pick the right system to test — and use the freeze-frame data stored with it (engine speed, load, temperature at the moment it triggered) to narrow things down before spending money on parts.

The most-searched VW & Audi fault codes

P0300 · P0301–P0304 — Cylinder misfire

Meaning: the engine misfired — a combustion event was weak or missing. P0300 is a random or multiple-cylinder misfire; P0301 to P0304 name the specific cylinder (1 to 4).

Common on VAG: worn spark plugs and failing coil packs are the usual suspects on TSI and TFSI engines. A boost or vacuum leak, a clogged injector, low fuel pressure, or carbon on the intake valves of direct-injection engines can all cause it too.

Read/clear: note which cylinder is flagged, check the freeze-frame for the load it happened under, swap suspect coils between cylinders to see if the misfire follows, then clear and re-test.

P0171 · P0174 — System too lean

Meaning: the ECU had to add a lot of fuel to keep the mixture correct, which tells it there is more air (or less fuel) than expected. P0171 is bank 1, P0174 is bank 2.

Common on VAG: unmetered air getting in — a split PCV/breather hose, a torn intake boot, a leaking intake manifold gasket, or a failing PCV diaphragm are classic on 1.8T, 2.0T and 2.0 TSI engines. A dirty MAF sensor or weak fuel supply can produce it as well.

Read/clear: look at the fuel-trim live data; large positive trims confirm the lean condition. Smoke-test the intake for leaks, then clear and watch the trims settle.

P0101 · P0102 — Mass air flow (MAF) sensor

Meaning: the MAF signal is implausible — either out of range (P0102, low input) or not tracking as expected (P0101, range/performance).

Common on VAG: a contaminated or aged MAF element, an air leak after the sensor, or a partly blocked air filter. MAF faults often show up alongside lean codes and a flat spot or hesitation under acceleration.

Read/clear: compare the MAF reading at idle and under load against a known-good figure; a lazy or low reading points at the sensor. Clear after cleaning or replacing.

P2015 — Intake manifold flap (runner) position sensor

Meaning: the position sensor for the intake manifold flaps is reading outside its plausible range, so the ECU cannot confirm the swirl/tumble flaps are where it commanded.

Common on VAG: one of the most-searched VAG codes for a reason — on 2.0 TDI common-rail engines the plastic flap linkage or the actuator's position sensor wears or snaps. It is usually a manifold/actuator fault, not a wiring fault, and often needs the intake manifold (or its motor) replaced rather than a simple reset.

Read/clear: confirm with the actuator's live position value; clearing alone will not hold if the linkage is broken.

P0299 — Turbocharger underboost

Meaning: the engine made less boost than the ECU asked for. On many VAG cars this also drops the car into limp mode.

Common on VAG: a boost leak is the number-one cause — a split intercooler pipe, a popped-off charge-pipe clamp, or a cracked hose. A sticking wastegate or VNT actuator, a faulty N75 solenoid, a leaking diverter valve, or a tired turbo can also trigger it.

Read/clear: compare requested versus actual boost in live data, pressure-test the charge pipes for leaks, then clear and road-test.

P0234 — Turbocharger overboost

Meaning: the opposite of P0299 — boost ran higher than the target. The ECU cuts power to protect the engine.

Common on VAG: a stuck or slow wastegate/VNT mechanism, a sticking N75 valve, or a disconnected wastegate actuator hose. On modified cars it can also appear if a boost target is set higher than the ECU's ceiling.

Read/clear: watch actual boost against the limit in live data, check the wastegate actuator moves freely, then clear and re-test.

P0011 · P0021 — Camshaft timing over-advanced

Meaning: the variable valve-timing system moved the camshaft further than commanded, or could not hold the commanded position. P0011 is bank 1, P0021 is bank 2.

Common on VAG: oil-related, almost always. A dirty or failing cam-adjuster (VVT) solenoid, low or degraded oil, a clogged oil-control screen, or a worn cam-phaser on TSI/TFSI engines. Neglected oil changes make this one much more likely.

Read/clear: check oil level and condition first, inspect the VVT solenoid, then clear and confirm the cam-timing deviation returns to zero in live data.

P0016 — Crankshaft / camshaft correlation

Meaning: the crankshaft and camshaft position signals are out of sync — the ECU sees the cam in the wrong place relative to the crank.

Common on VAG: on chain-driven TSI/TFSI engines this is a well-known warning sign of a stretched timing chain or a failed chain tensioner. It can also be a worn cam follower, a VVT fault, or a crank/cam sensor issue. Take it seriously — a jumped chain can destroy the engine.

Read/clear: do not just clear and drive. Inspect the chain/tensioner and camshaft timing before returning the car to service.

P0087 — Fuel rail pressure too low

Meaning: the high-pressure fuel system could not build or hold the pressure the ECU requested.

Common on VAG: a worn high-pressure pump (the cam-follower wear issue on early 2.0 TSI/TFSI is classic), a failing low-pressure lift pump in the tank, a clogged fuel filter, or a leaking pressure regulator. On TDIs a tired HPFP or contaminated fuel can cause it.

Read/clear: compare specified and actual rail pressure in live data under load; a large shortfall confirms it. Clear only after the supply issue is fixed.

P0420 · P0430 — Catalytic converter efficiency below threshold

Meaning: the rear oxygen sensor is mirroring the front one too closely, so the ECU concludes the catalyst is no longer cleaning the exhaust well enough. P0420 is bank 1, P0430 is bank 2.

Common on VAG: often a genuinely aged catalyst, but not always — a lazy or contaminated rear oxygen sensor, an exhaust leak upstream of the sensor, or an unresolved misfire or rich/lean condition can all fake a "bad cat" reading.

Read/clear: rule out sensor and exhaust-leak causes first (they are far cheaper), compare the front and rear sensor traces in live data, then clear and re-run a drive cycle.

P0401 — EGR insufficient flow (diesel)

Meaning: the exhaust-gas recirculation system is flowing less than expected.

Common on VAG: a soot-clogged EGR valve or cooler, or a sticking EGR valve, on TDI engines. Carbon build-up in the intake tract is the usual root cause, and it often comes with reduced power and rough running.

Read/clear: inspect and clean or replace the EGR valve, check for intake carbon, then clear and confirm the fault does not return under load.

P0442 · P0455 · P0456 — EVAP system leak

Meaning: the evaporative-emissions system, which captures fuel vapour from the tank, is leaking. P0455 is a large leak, P0442 a medium one, P0456 a very small one.

Common on VAG: the cheapest and most common cause is a loose, worn or missing fuel cap. Beyond that: a cracked charcoal-canister purge line, a failed purge or vent valve, or a perished vapour hose.

Read/clear: re-seat the fuel cap first, then check the purge valve and hoses. Clear the code — these often need a few complete drive cycles before the readiness monitor re-runs and confirms the fix.

How to read and clear these codes properly

Reading a code is only step one. To actually diagnose a VW or Audi fault:

• Read every module, not just the engine. A dash warning can come from ABS, airbag, transmission or comfort modules — a full scan finds all of them.
• Capture the freeze-frame. The conditions stored with the code (speed, load, temperature) tell you when the fault happens, which is often more useful than the code itself.
• Watch live data. Fuel trims, requested-versus-actual boost, cam-timing deviation and rail pressure turn a vague code into a specific diagnosis.
• Fix first, clear second. Clearing before repairing just hides the problem; the ECU re-detects it. Clear after a repair to confirm it stays gone.

Which tools can read, clear and go further

Almost any scan tool can read and clear these codes. Where VAG tools differ is how deep they go — and whether they can also read your ECU, back it up and tune it.

Competitor capabilities change — check current specs before buying. VCDS has the deepest label and fault-code coverage in the VAG world and remains the benchmark for diagnostics; the difference here is ECU reading and tuning, not code-reading.

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