P21BF: Cold Start Cylinder 10 Injection Timing

Cylinder 10 is the last cylinder of a V10 and exists otherwise only on twelve-cylinder engines. V10 engines are rare: Ford's 6.8-litre truck V10 (firing order 1-6-5-10-2-7-3-8-4-9, in the E-Series until 2019) and the 5.2-litre direct-injection V10 of Lamborghini and Audi. P21BF states that cylinder 10, during the cold-start heating phase, received its fuel at a different moment than targeted. No manufacturer documents the code publicly; the description rests on CARB's cold-start monitoring rules and on the system code P05EC, which GM checks across all cylinders of its diesels. If P021D (warm timing) or P0210 (injector circuit) are stored too, the cylinder 10 injector is the prime suspect; a cold-start pulse code does not exist for this cylinder.

Cylinder 10 is numbered according to the manufacturer: check your engine's manual before working on it.

Symptoms

• Check engine light after one or more cold starts
• Uneven idle until the engine warms up

Most likely causes

• Injector of cylinder 10 aged or contaminated
• Wiring or connector problem at injector 10
• Engine module software needing a cold-start calibration update

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the connector and wiring of the injector indicated
2. Measure the resistance and compare it with the manufacturer's specification
3. Check the control signal with an oscilloscope; on direct injection or diesel engines, leave this to a repair shop because of the high voltages
4. On diesels, check the per-cylinder injection corrections

Good to know

• It affects emissions.

What each character of P21BF means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 1: Air and fuel
• BF: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21BF?

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 P21BF?

The most common causes are: injector of cylinder 10 aged or contaminated; wiring or connector problem at injector 10; engine module software needing a cold-start calibration update.

Related codes

• P21BECold Start Cylinder 9 Injection Timing
• P21C0Cold Start Cylinder 11 Injection Timing
• P021DCylinder 10 Injection Timing
• P0210Cylinder 10 Injector A Circuit/Open
• P0310Cylinder 10 Misfire Detected
• P05ECCold Start Injection Timing Performance

Sources

• law.cornell.edu: 13 CCR 1968.2
• en.wikipedia.org: Ford Modular engine
• en.wikipedia.org: Lamborghini V10 engine
• gsi.ext.gm.com: 25OBDG04A

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.