P21D7: Cylinder 9 Injector B Circuit/Open

P21D7 would mark an open drive circuit of the second injector on cylinder 9, which presupposes at least nine cylinders. No manufacturer documents this code publicly; the published Ford and GM lists stop at cylinder 8 (P21D6). Engines with that many cylinders and two injection systems do exist, for example the V10 of the Lamborghini Huracán, which pairs direct with multipoint injection, but no source shows how its maker labels the two injectors. CARB requires that electrical output components such as fuel injectors be checked for circuit faults, so the logic would follow the documented siblings: a missing current or excessive impedance when the injector is switched. The direct injector of cylinder 9 would carry P0209 if it is the A side. A misfire on cylinder 9 (P0309) is the likely accompanying symptom.

Medium severity. Can I keep driving?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.

Cylinder 9 is numbered according to the manufacturer: check your engine's manual before working on it.

Symptoms

Check engine light on
Misfire on cylinder 9 (P0309)

Most likely causes

Open coil in the second injector of cylinder 9
Broken wire or terminal in the injector harness
Driver stage in the engine control module failed

What to check, in order

Start with the simplest. You can tick off each step.

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

Good to know

Repair difficulty: hard
It affects emissions.

What each character of P21D7 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
1: Air and fuel
D7: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21D7?

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 P21D7?

The most common causes are: open coil in the second injector of cylinder 9; broken wire or terminal in the injector harness; driver stage in the engine control module failed.

Related codes

Sources

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.