P21A1 on Audi
Cylinder 6 Air/Fuel Ratio Imbalance
P21A1 on Audi vehicles indicates cylinder 6 air/fuel ratio imbalance. Au7o has documented this code across 2 Audi models — most commonly on Q5, SQ5. Generic SAE J2012 code. Sets when the PCM detects that cylinder 6's air/fuel mixture is imbalanced (too rich or too lean) relative to the other cylinders on its bank. Per-cylinder contribution is inferred from the upstream oxygen/air-fuel sensor signal together with crankshaft and camshaft position data. Applies to engines with six or more cylinders and indicates an imbalance rather than a misfire.
Common Causes of P21A1
- •Clogged or leaking fuel injector on cylinder 6
- •Intake / vacuum leak near cylinder 6
- •Fouled spark plug or weak ignition coil on cylinder 6
- •Faulty oxygen / air-fuel ratio sensor
- •Contaminated MAF sensor
- •Low or inconsistent fuel pressure
- •Low compression on cylinder 6
P21A1 on Audi by Model
Audi Q5(1 issue)
- 2009-2025 Q5 Gasoline-Quality Deposit Diagnosis - TSB 2014753/132009-2025
Audi states that contaminated gasoline or gasoline with insufficient deposit-control additives can contribute to deposits on intake valves, the intake manifold, fuel injectors and combustion chambers. Possible results include rough running after cold start, extended cranking, hesitation, rough idle, reduced performance, poor fuel economy or an MIL with misfire, lean-system or air/fuel-ratio-imbalance faults. This does not establish a universal Q5 intake-valve defect, a mandatory walnut-blasting interval or a fixed failure mileage.
Audi SQ5(1 issue)
- Fuel-Quality or Deposit-Related Rough Running and Misfire Diagnosis2014-2025
Audi states that contaminated gasoline or gasoline with insufficient deposit-control additives can contribute to deposits in injectors, combustion chambers, the intake manifold and intake valves, with rough running, hesitation, reduced performance, poor fuel economy or related ECM faults. This does not establish a universal SQ5 intake-valve defect, a mandatory walnut-blasting interval or a fixed mileage. Audi separately warns that unapproved fuel additives are not permitted on 2018-2025 EA839 V6 TFSI SQ5 models because a negative effect on engine life cannot be ruled out.
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View P21A1 across all makes →Frequently Asked Questions
What does P21A1 mean on Audi?▼
P21A1 stands for "Cylinder 6 Air/Fuel Ratio Imbalance." Generic SAE J2012 code. Sets when the PCM detects that cylinder 6's air/fuel mixture is imbalanced (too rich or too lean) relative to the other cylinders on its bank. Per-cylinder contribution is inferred from the upstream oxygen/air-fuel sensor signal together with crankshaft and camshaft position data. Applies to engines with six or more cylinders and indicates an imbalance rather than a misfire. On Audi specifically, this code is documented across 2 models.
What causes P21A1 on Audi vehicles?▼
Common causes on Audi: Clogged or leaking fuel injector on cylinder 6, Intake / vacuum leak near cylinder 6, Fouled spark plug or weak ignition coil on cylinder 6, Faulty oxygen / air-fuel ratio sensor, Contaminated MAF sensor. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P21A1 on a Audi?▼
Repair costs vary widely depending on the root cause and specific Audi model.
Which Audi models have P21A1 documented?▼
Au7o has documented P21A1 on 2 Audi models: Q5, SQ5.