P21A1: Cylinder 6 Air/Fuel Ratio Imbalance
2 vehicles · 1 makes
Most Reported On
P21A1 is an OBD-II diagnostic trouble code meaning “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. This code is most commonly reported on 2014-2025 Audi SQ5, and 2009-2025 Audi Q5.
Common Causes
Diagnostic Tools
This is a powertrain code, so a basic OBD-II reader can read it. These read P21A1.
ANCEL AD310 Classic OBD-II Scanner
by ANCEL
A simple code reader for engine (P) codes on many compatible OBD-II vehicles. It does not support every vehicle or hybrid/EV application, and it does NOT read body, chassis or network codes; confirm exact-vehicle compatibility before buying.
- Read & clear engine (P) codes
- View freeze frame data
- I/M readiness status
- No batteries or app needed
BlueDriver Pro Bluetooth Scanner
by BlueDriver
Bluetooth scanner with a companion app that reads beyond the engine module — ABS, SRS and transmission — plus manufacturer-specific codes on many vehicles.
- Enhanced diagnostics (ABS, SRS, transmission)
- Repair Reports with verified fixes
- Smog check readiness
- Free app (iOS & Android)
LAUNCH CRP123X V3.0 Elite OBD-II Scanner
by LAUNCH
Handheld scanner covering the four major systems (engine, transmission, ABS, SRS) with live data streaming and graphing.
- Engine, transmission, ABS, SRS diagnostics
- Live data stream & graphing
- AutoVIN for vehicle identification
- Free lifetime updates via Wi-Fi
Autel MaxiCOM MK808S Diagnostic Tool
by Autel
Shop-level tablet with all-system coverage and bi-directional control, so you can command a module rather than only read it.
- All-system diagnostics (25+ modules)
- Bi-directional control & active tests
- Oil reset, EPB, BMS, TPMS, injector coding
- 7-inch touchscreen with Android OS
What you can do yourself, and what needs a shop
Reading the code, clearing it, and watching whether it returns is a driveway job with the right scanner. These steps are not:
- Module programming or software updates — needs the manufacturer software or a J2534 pass-through with a paid subscription.
- Replacing a control module — most are VIN-programmed and immobilizer-registered, so a new one does nothing until it is coded to your car.
- Airbag/SRS component work — a stored fault can deploy a live device.
If the vehicle is still under warranty, check for an open recall or technical service bulletin before buying anything — the repair may already be covered.
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FAQ
What does P21A1 mean?
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.
What are the most common causes of P21A1?
The most common causes of P21A1 are: 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. The specific cause varies by vehicle.
How much does it cost to fix P21A1?
Repair costs vary widely depending on the root cause and vehicle.
Which vehicles are affected by P21A1?
Au7o has documented P21A1 across 2 vehicle models from 1 manufacturers: Audi.
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.