P21A0

Code

Generic P — Powertrain

Cylinder 5 Air-Fuel Ratio Imbalance

Brand: Generic

Type: P — Powertrain

Views: UK: 39 EN: 110 RU: 43

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Causes

Clogged, leaking or stuck fuel injector on cylinder 5
Open/shorted injector wiring or poor connector/ECU driver circuit
Intake manifold gasket or vacuum leak localized at cylinder 5
Low compression or valve sealing problem on cylinder 5
Faulty or contaminated air–fuel (lambda/O2) sensor or cylinder-specific A/F sensor
Fuel delivery problem (fuel rail pressure, regulator) affecting one injector

Symptoms

Malfunction Indicator Lamp (MIL) illuminated
Rough idle or vibration, especially at idle
Single-cylinder misfire or intermittent misfire codes (may show P0305)
Reduced fuel economy or drivability issues under load
Hesitation, stumble or lack of power from the engine
Possible fuel smell if cylinder running rich

What to check

Read and record all stored/ pending codes and freeze-frame data
Compare live A/F (lambda) or O2 sensor data and fuel trims for affected bank/cylinder
Check for related misfire (P0305) or injector circuit (P0205) trouble codes
Visually inspect injector harness/connector for corrosion, loose pins, damaged insulation
Measure injector resistance and compare with spec; inspect for carbon/fuel deposits
Command injector on/off with scan tool or use a noid light/oscilloscope to verify pulse

Signal parameters

O2 sensor voltage (narrowband): ~0.1–0.9 V switching; sluggish or biased readings indicate issues
Wideband AFR: stoichiometric ≈ 14.7:1 (lambda ≈ 1.0); cylinder 5 should track other cylinders
Short-term fuel trim (STFT): typically within ±10% per bank; large deviation on affected side is notable
Long-term fuel trim (LTFT): should be stable and within ±10% for normal operation
Injector resistance: vehicle-specific (typical common range 10–16 Ω); compare to spec
Injector pulse width at idle: typically ~1–4 ms (varies by engine/load); abnormal values or no pulses indicate control/wiring fault

Diagnostic algorithm

Retrieve freeze-frame and live data. Note A/F or O2 readings and per-bank fuel trims when code set; confirm code reproducibility.
Scan for related codes (P0305, P0205, P0171/P0172, P0174). Address these as applicable before deeper tests.
Visually inspect injector connector and wiring to cylinder 5. Wiggle test while monitoring injector command and response.
Check injector supply voltage and ground, measure injector coil resistance. If resistance out of spec or intermittent, replace or bench-test injector.
Command injector with scan tool (if supported) and confirm fuel delivery (fuel smell, fuel pressure drop, or use a flow/volume test).
Swap the cylinder 5 injector with another known-good injector (same type) and clear codes. If the code follows the injector to the other cylinder, replace the injector.
If swapping doesn’t move the fault, inspect and test wiring/driver circuit (noid light/oscilloscope) and check PCM driver outputs.
Perform intake smoke test around cylinder 5 intake runner and gasket to find leaks that would cause a local lean condition.
Perform compression and/or leak-down test on cylinder 5 to rule out mechanical sealing issues causing irregular A/F behavior.
Check fuel rail pressure and regulator; confirm adequate static and dynamic pressure and no air in fuel.
Inspect and test the A/F or O2 sensor(s); compare upstream/downstream sensor signals and response times. Replace if sensor readings are biased or sluggish.
After repairs or parts replacement clear codes and road-test with live data capture to confirm cylinder 5 A/F returns to nominal and code does not recur.

Likely causes

Faulty/inconsistent fuel injector performance (most common)
Wiring/connector fault to injector or local sensor
Intake manifold leak or runner gasket near cylinder 5
Cylinder mechanical issue (compression/leakdown)
Faulty local A/F/O2 sensor or inaccurate sensor reading

Fault status

⚠️ Status

Cylinder 5 air–fuel ratio imbalance detected — PCM has recorded a persistent deviation for cylinder 5 compared with other cylinders/expected A/F values.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5 - 3.5 hours

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