P21B6: Cold Start Cylinder 1 Injection Timing

What does P21B6 mean?

After a cold start, petrol direct-injection and diesel engines deliberately move the moment of injection – for example with an extra late pulse – so that more heat reaches the catalyst. CARB 13 CCR 1968.2 (e)(11) wants every element of this strategy checked while it runs and reported apart from warm-engine faults. P21B6 pins such a deviation on cylinder 1. No manufacturer documents this code publicly; GM's 2025 diesel tables compare requested and scheduled start of injection on every cylinder but store only the system code P05EC. If cylinder 1 alone is flagged, a shared cause such as crank/cam synchronisation is unlikely; the injector, its wiring or its driver stage come first. Warm counterpart: P020A.

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
• Slightly rough idle in the first minute after a cold start
• No noticeable fault once the engine is warm

Common causes

• High Injector of cylinder 1 responds late or inconsistently when cold
• Medium Corroded or loose connector or harness at injector 1
• Low Injector driver stage in the engine control module faulty
• Low Engine module software with an overly strict cold-start check

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