P21A3

Code

Generic P — Powertrain

Cylinder 8 Air-Fuel Ratio Imbalance

Brand: Generic

Type: P — Powertrain

Views: UK: 37 EN: 56 RU: 37

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Page language: EN

Causes

Faulty or leaking fuel injector (stuck open, partially clogged, leaking at pintle)
Injector wiring harness or connector fault (open, short, high resistance, poor pin contact)
Faulty local oxygen/air-fuel sensor or wideband sensor reading biased for that cylinder
Intake manifold gasket leak or localized vacuum leak at cylinder 8 intake port
Compression or mechanical issue on cylinder 8 (low compression, valve sealing, head gasket)
Carbon/fuel fouling or oil contamination of injector or intake port

Symptoms

Check Engine Light (CEL) illuminated with corresponding stored code
Rough idle or vibration at idle
Cylinder-specific misfire or reduced power under load
Increased fuel consumption and poor fuel economy
Black smoke from exhaust or strong fuel smell if running rich on that cylinder
Failed emissions test or high HC/CO readings

What to check

Scan for all stored/pendant codes and view freeze-frame data (ignition, coolant temp, rpm, fuel trims)
Capture live data: short-term and long-term fuel trims, per-cylinder fuel trims (if available), O2/wideband sensor readings
Check misfire counters and cylinder contribution tests to see if cylinder 8 is flagged
Visual inspection of fuel injector connector, wiring harness, and ground connections for damage or corrosion
Inspect intake manifold and gasket area around cylinder 8 for vacuum leaks or gasket failure
Measure fuel rail pressure at idle and under load to confirm system pressure and regulator operation

Signal parameters

Short-term fuel trim (STFT) and long-term fuel trim (LTFT): expect values near 0%; significant positive LTFT (+10% to +25%) suggests lean condition on that side; significant negative LTFT (−10% to −25%) suggests rich condition
Per-cylinder fuel trim or injector pulse-width variation (if offered by scanner) showing cylinder 8 deviating from others
Wideband O2 sensor lambda ~1.0 at stoich; values >1.05 indicate lean,
Narrowband O2 voltage switching ~0.1–0.9 V (fast switching around stoich) — a biased/stuck sensor may read steady high or low
Fuel rail pressure: nominal range depends on vehicle (examples: 40–70 psi for port-injected gas engines); watch for pressure drops during load
Injector waveform: consistent pulse width and clean square wave drive from PCM; look for abnormal rise/fall or missing pulses

Diagnostic algorithm

Retrieve trouble codes, freeze-frame and live data. Note related codes (misfires, O2 sensor faults, fuel trim anomalies).
Confirm symptom: monitor STFT/LTFT and per-cylinder metrics (contribution test/misfire counts). Verify cylinder 8 shows AFR imbalance relative to others.
Visually inspect injector connector, wiring harness, and ground for damage/corrosion. Wiggle while monitoring for changes in live data.
Check fuel pressure and regulator operation to rule out low system pressure or pressure fluctuation.
Measure injector electrical resistance at cylinder 8 and compare to other injectors. If abnormal, replace or bench-test the injector.
Use a scope or noid light to verify injector drive signal (pulse width, frequency, amplitude). If no drive or odd waveform, suspect PCM driver or wiring.
Perform a balance test (disable injector or use current clamp) or swap injector from cylinder 8 with another cylinder. If imbalance follows injector, replace it; if it follows position, suspect wiring/PCM/air leak.
Inspect for intake manifold or vacuum leaks at cylinder 8 intake port (smoke test or propane/aerosol while monitoring fuel trims).
Conduct compression and/or leak-down test on cylinder 8 and compare with other cylinders to identify mechanical issues.
Check upstream O2/wideband sensor operation and placement. Replace sensor if out of spec or if sensor bias is causing false AFR reading.
After repairs or part replacement, clear codes and road-test while monitoring fuel trims, per-cylinder data, and O2 readings to confirm the imbalance is resolved.
If tests indicate PCM or injector driver failure, consider module bench testing or replacement per manufacturer procedures.

Likely causes

Faulty or partially clogged fuel injector on cylinder 8
Injector electrical problem (connector corrosion, broken wire, high resistance)
Intake manifold vacuum leak localized to cylinder 8
Faulty or biased oxygen/air-fuel sensor (wideband) affecting cylinder sampling

Fault status

⚠️ Status

Cylinder 8 air–fuel ratio imbalance detected — PCM has logged a persistent deviation in AFR for cylinder 8 compared with expected values or other cylinders. Investigate injector, wiring, intake leak, sensors and mechanical condition.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

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