P21BD: Cold Start Cylinder 8 Injection Timing

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.

Cylinder 8 is numbered according to the manufacturer: check your engine's manual before working on it.

Symptoms

• Check engine light after cold starts
• Rough running or smoke in the first minute after a cold start
• No symptoms at operating temperature

Most likely causes

• Injector of cylinder 8 with slow or varying response
• Connector or wiring fault at injector 8
• Cold-start calibration in the engine module needing an 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

• Repair difficulty: hard
• It affects emissions.

What each character of P21BD means

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

Frequently asked questions

Can I keep driving with code P21BD?

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 P21BD?

The most common causes are: injector of cylinder 8 with slow or varying response; connector or wiring fault at injector 8; cold-start calibration in the engine module needing an update.

Related codes

• P21BCCold Start Cylinder 7 Injection Timing
• P21BECold Start Cylinder 9 Injection Timing
• P021BCylinder 8 Injection Timing
• P2B0FCold Start Cylinder 8 Injection Pulse Performance
• P0208Cylinder 8 Injector A Circuit/Open
• P0308Cylinder 8 Misfire Detected
• P05ECCold Start Injection Timing Performance

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 25OBDG04A
• static.nhtsa.gov: MC 10143438 9999

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.