P21B8

Code

Generic P — Powertrain

Cold Start Cylinder 3 Injection Timing

Brand: Generic

Type: P — Powertrain

Views: UK: 30 EN: 37 RU: 29

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Page language: EN

Causes

Faulty or sticking fuel injector on cylinder 3
Injector driver circuit fault (open, short, intermittent)
Incorrect injector timing due to cam/crank position correlation issue
Faulty camshaft or crankshaft position sensor or reluctor/trigger wheel damage
Low or inconsistent fuel pressure during cold start (pump, regulator, leaks)
Engine coolant temperature (ECT) or intake air temperature (IAT) sensor faults causing incorrect cold-start enrichment/timing

Symptoms

Difficult or no start when engine is cold
Rough idle, misfire or vibration immediately after cold start
Excessive white/black smoke on cold start (diesel: white/black smoke; gasoline: rich stumble)
Reduced cold-start drivability and increased emissions until engine warms
MIL (check engine light) illuminated; possible limp-home behavior depending on vehicle

What to check

Scan for related codes and freeze-frame data; note engine temperature and conditions when the fault set
Attempt to reproduce fault under cold-start conditions while connected to a scan tool with live data
Inspect wiring and connectors for injector #3, cam/crank sensors, and related grounds for corrosion, damage, or loose pins
Measure injector #3 electrical parameters (resistance, continuity) and compare to other injectors and spec
Capture injector drive waveform with oscilloscope or lab scope during a cold start to verify pulse timing and shape
Check camshaft and crankshaft sensor waveforms for correct timing relationship and missing or noisy edges

Signal parameters

Injector command pulse timing (crank degrees) vs actual injector event — should match ECU command within manufacturer tolerance
Injector pulse width during cold start — increased vs warm, but consistent with other cylinders under same conditions
Injector resistance/impedance — compare to service specification and to other injectors
Injector driver voltage/current waveforms — clean switching, no excessive dwell or voltage drop
Crankshaft and camshaft position sensor waveforms — stable amplitude and timing relationship at cranking speed
Fuel rail pressure at cranking/cold start — within manufacturer specified cold-start range

Diagnostic algorithm

Retrieve DTCs, freeze frame, and live data. Confirm P21B8 and any related or pending codes (injector circuit, cam/crank sensors, fuel pressure, temperature sensors).
Verify vehicle condition: reproduce with a cold start (follow safety and emissions guidelines). Monitor injector timing data and related sensor inputs in live data.
Visual inspection: check wiring harness, connectors, pins and grounds for injector #3, ECT/IAT, cam/crank sensors, and main power/ground feeds to the ECU and fuel pump.
Measure injector #3 resistance and compare to service spec and the other injectors. If out of range, replace or bench test the injector.
Using a lab scope, capture injector #3 drive waveform and compare timing/shape to another cylinder. Look for delayed switching, long dwell, missing pulses or low drive voltage.
Verify crank and cam sensor waveforms and the cam-to-crank correlation at cranking speed to rule out mechanical timing or sensor issues.
Check fuel rail pressure during initial cranking and cold start; repair fuel pump, lines, regulator, or check for leaks if pressure is low or inconsistent.
Confirm ECT and IAT sensor readings are accurate (use secondary thermometer). Replace sensor if readings are incorrect and clear codes.
If electrical tests are inconclusive, swap injector #3 with another cylinder’s injector (if practical) to see if fault follows the injector or remains at cylinder position.
If fault follows the injector, replace or service the injector. If fault remains at cylinder position after swap, investigate wiring back to ECU, driver transistor, or mechanical timing/compression issues.
After repairs, clear codes and perform multiple cold-start cycles to confirm the fault does not return and verify drivability.

Likely causes

Injector 3 electrical circuit intermittent or connector corrosion
Injector 3 stuck open/partial drip or slow response when cold
Cam/crank sensor signal misalignment at cold crank (timing slip or sensor fault)
Low fuel rail pressure on initial cranking due to pump or pressure regulator
Incorrect ECT/IAT input making ECU command incorrect for cold conditions

Fault status

⚠️ Status

Stored when the ECU detects the injection timing/event for cylinder 3 during cold-start conditions is outside expected parameters. MIL may be illuminated; drivability and emissions can be affected until repaired.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

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