P21B9 Cold Start Cylinder 4 Injection Timing
What does P21B9 mean?
P21B9 — Cold Start Cylinder 4 Injection Timing. This is an OBD-II diagnostic trouble code a scanner retrieves when your vehicle's computer detects cold start cylinder 4 injection timing. The level of risk depends on how the vehicle is currently running and how long you've been driving with the light on.
Common causes of P21B9
- Faulty or contaminated sensor in the fuel/air metering circuit
- Damaged or shorted wiring or connector for the affected circuit
- Vacuum leak or intake system restriction
- Failing fuel pump, injector, or fuel pressure regulator
How to fix P21B9
These are general steps for this system. The exact cause depends on your vehicle — confirm with live scan data before replacing parts.
- Read freeze frame data and related codes with an OBD-II scanner
- Inspect the affected sensor wiring and connector for damage
- Compare live sensor readings (fuel trim, airflow, temperatures) to manufacturer specs
- Smoke test the intake system if lean or vacuum conditions are suspected
- Replace the failed component after confirming it with live data