Cylinder 6 exists on six-cylinder engines and anything larger – inline sixes and V6 units such as Ford's 3.5-litre EcoBoost petrol or the 3.0-litre EcoDiesel in the Ram 1500. For that diesel FCA listed the warm-engine timing codes P020A–P020F together with the over-advanced and over-retarded codes in a 2021 bulletin whose fix was reprogramming the engine module. P21BB is the cold-start version for cylinder 6: during the heating phase after start, its injection timing did not match the target. Diesels adapt injection timing to coolant temperature after a cold start to hold emissions down before NOx sensors are working, as a GM patent describes. No manufacturer documents P21BB publicly; a software update is worth checking before replacing parts.
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 6 is numbered according to the manufacturer: check your engine's manual before working on it.
Symptoms
- Check engine light after cold starts
- On diesels, more white smoke or knocking in the first minutes
- Hard start or rough idle when cold
Most likely causes
- Injector of cylinder 6 worn, coked or with drifting opening delay
- Outdated engine module software (analogous timing codes were cured by reprogramming)
- Damaged wiring or connector at injector 6
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 P21BB means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 1: Air and fuel
- BB: Specific fault in that system
Frequently asked questions
Can I keep driving with code P21BB?
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 P21BB?
The most common causes are: injector of cylinder 6 worn, coked or with drifting opening delay; outdated engine module software (analogous timing codes were cured by reprogramming); damaged wiring or connector at injector 6.
Related codes
- P21BACold Start Cylinder 5 Injection Timing
- P21BCCold Start Cylinder 7 Injection Timing
- P020FCylinder 6 Injection Timing
- P2B0DCold Start Cylinder 6 Injection Pulse Performance
- P0206Cylinder 6 Injector A Circuit/Open
- P0306Cylinder 6 Misfire Detected
- P05ECCold Start Injection Timing Performance
Sources
- law.cornell.edu: 13 CCR 1968.2
- static.nhtsa.gov: MC 10220589 9999
- patents.google.com: en
- en.wikipedia.org: Ford EcoBoost engine
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.