P21BF: Cold Start Cylinder 10 Injection Timing

What does P21BF mean?

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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

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

Common causes

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

Likelihood is a general estimate across vehicles; your make and model may differ.