P21B9: Cold Start Cylinder 4 Injection Timing

What it means

Cylinder 4 cold-start injection timing is outside the calibrated range. The code does not by itself identify the cause.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The engine control module calculates the injection event for cylinder 4 during cold starting using crankshaft and camshaft position and engine conditions. This code sets when cylinder 4's cold-start injection timing is outside its calibrated range, which may result from injector flow or control, fuel delivery, or timing-signal issues.

Common causes

Most likely first.

1. Cylinder 4 injector wear or altered flow
2. Injector wiring or control fault
3. Fuel pressure or delivery problem
4. Crank or cam timing signal issue

Symptoms

• Check-engine light
• Rough running or misfire after a cold start
• Hard starting when the engine is cold
• Increased exhaust smoke

How to diagnose it

1. Save freeze-frame data and check cylinder 4 cold-start timing and misfire data during a cold start, noting coolant temperature and fuel pressure.
2. Inspect the cylinder 4 injector wiring and connector for loose pins, corrosion, chafing, or heat damage.
3. Check for related injector, fuel-pressure, crankshaft, camshaft, or correlation codes that could identify a shared fault.
4. Compare cylinder 4 injector correction or balance data against the other cylinders under similar cold conditions.
5. Test cylinder 4 injector flow and control, then verify crank-to-cam signals and mechanical timing if needed.

Common fixes

• Repair damaged or poorly connected cylinder 4 injector wiring.
• Correct a fuel-pressure or delivery problem confirmed by testing.
• Service or replace the cylinder 4 injector if its flow or control is out of specification.
• Repair a crankshaft or camshaft signal fault, or correct verified mechanical timing error.

Can I keep driving?

Gentle, limited driving may be possible if the engine runs smoothly, but arrange diagnosis soon because cold-start misfire and smoke can increase emissions and may worsen. Avoid driving if the misfire is severe, the warning flashes, or starting becomes unreliable.