P21D8: Cylinder 10 Injector B Circuit/Open

P21D8 covers the second injector of cylinder 10, the last cylinder of a V10. No manufacturer documents this code publicly. A V10 with two injectors per cylinder is on record in the Lamborghini Huracán, whose engine combines direct and multipoint injection, yet which of them its software would call B is unknown. For comparison, Ford's eight-cylinder logic tests B circuits continuously above 11 V for 5 s, and GM's LT5 module rates an open by 200 kOhm or more with the driver off. CARB obliges makers to monitor injector output circuits whose failure can raise emissions. In practice the fault would show as a misfire on cylinder 10 (P0310) with that cylinder still fed by its other injector; the A-side equivalent would be P020A. Neighbours are P21D7 and P21D9.

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

Symptoms

• Check engine light on
• Misfire on cylinder 10 (P0310)

Most likely causes

• Second injector of cylinder 10 open internally
• Wiring or connector fault at the cylinder 10 injector
• Output stage in the engine control module failed

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. Check the continuity of the wire between the component and the control module.
3. Measure the resistance and compare it with the manufacturer's specification
4. Check the control signal with an oscilloscope; on direct injection or diesel engines, leave this to a repair shop because of the high voltages
5. On diesels, check the per-cylinder injection corrections

Good to know

• Repair difficulty: hard
• It affects emissions.

What each character of P21D8 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 1: Air and fuel
• D8: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21D8?

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 P21D8?

The most common causes are: second injector of cylinder 10 open internally; wiring or connector fault at the cylinder 10 injector; output stage in the engine control module failed.

Related codes

• P020ACylinder 1 Injection Timing
• P21EECylinder 10 Injector B Circuit Low
• P21EFCylinder 10 Injector B Circuit High
• P21D7Cylinder 9 Injector B Circuit/Open
• P21D9Cylinder 11 Injector B Circuit/Open
• P0310Cylinder 10 Misfire Detected

Sources

• law.cornell.edu: 13 CCR 1968.2
• en.wikipedia.org: Lamborghini Huracán
• gsi.ext.gm.com: 19OBDG03A Diagnostics

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.