P21BDCold Start Cylinder 8 Injection Timing
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- P21BD covers cylinder 8, the last cylinder of a V8. GM's 2025 diesel tables show how close an existing monitor comes to this code: during cold start the module compares requested and scheduled start of injection, with every cylinder from A to H included in the enable checks, and a deviation beyond ±4 in 88 of 176 samples sets the lamp in the first trip. Yet GM stores only the system code P05EC, which does not say which injector misbehaved. A per-cylinder code like P21BD would point straight at cylinder 8; no manufacturer documents its use publicly. GM's petrol V8 with direct injection likewise use a special dual-pulse cold-start mode. Related: P021B (warm), P2B0F (cold-start pulse), P0208, P0308.
- What you may notice
- Check engine light after cold starts
- Rough running or smoke in the first minute after a cold start
- No symptoms at operating temperature
- Likely causes
- LikelyInjector of cylinder 8 with slow or varying response
- PossibleConnector or wiring fault at injector 8
- RareCold-start calibration in the engine module needing an update
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €120 – €1,600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.