Four-cylinder petrol direct injection is the most widespread engine type this code can apply to, and cylinder 4 is its last cylinder. Ford's 2011 OBD summary for its first turbo direct-injection engine explains the background: during catalyst heating the injection timing may be specific to that mode, possibly as two injections with one during the compression stroke, which in turn needs a rail pressure window (checked as P053F). P21B9 reports that this cold-start timing went wrong on cylinder 4 alone. No manufacturer lists the code publicly, so the description rests on CARB's cold-start rules and the documented system codes. Warm-engine counterpart: P020D; electrical injector fault: P0204; cold-start pulse: P2B0B.
Medium severity. Can I keep driving?Drive as little as possible and go to a repair shop. If you smell fuel or the engine is shaking badly, do not keep driving.
Cylinder 4 is numbered according to the manufacturer: check your engine's manual before working on it.
Symptoms
- Check engine light, usually without drivability complaints
- Slightly longer catalyst warm-up and possibly higher cold-start emissions
- Occasional hesitation right after a cold start
Most likely causes
- Deposits on the tip of injector 4 changing its opening behaviour
- Harness fault on the injector 4 circuit
- Rail pressure barely inside its window during cold start, affecting the last cylinder first
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
- It affects emissions.
What each character of P21B9 means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 1: Air and fuel
- B9: Specific fault in that system
Frequently asked questions
Can I keep driving with code P21B9?
Drive as little as possible and go to a repair shop. If you smell fuel or the engine is shaking badly, do not keep driving.
What causes code P21B9?
The most common causes are: deposits on the tip of injector 4 changing its opening behaviour; harness fault on the injector 4 circuit; rail pressure barely inside its window during cold start, affecting the last cylinder first.
Related codes
- P21B8Cold Start Cylinder 3 Injection Timing
- P21BACold Start Cylinder 5 Injection Timing
- P020DCylinder 4 Injection Timing
- P2B0BCold Start Cylinder 4 Injection Pulse Performance
- P0204Cylinder 4 Injector A Circuit/Open
- P0304Cylinder 4 Misfire Detected
- P053FCold Start Fuel Pressure Performance Bank 1
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.