P21D9: Cylinder 11 Injector B Circuit/Open

What does P21D9 mean?

P21D9 is reserved for an open circuit of the second injector on cylinder 11 and therefore only makes sense on twelve-cylinder engines with dual injection. No manufacturer documents this code publicly, and no available source names a twelve-cylinder engine that uses it. Its meaning follows from the series: like P21CF to P21D6, it is an electrical check of the injector drive, not of fuel quantity. CARB requires such circuit monitoring for injectors whose failure affects emissions; Ford's documented B-injector check for up to eight cylinders runs continuously above 11 V and needs 5 s, which shows the typical scale. If it ever appears, check the injector coil resistance and the harness first. The direct counterpart in the A series is P020B, the misfire code for cylinder 11 is P0311.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on
• Misfire on cylinder 11 (P0311)

Common causes

• High Open coil in the second injector of cylinder 11
• Medium Damaged injector harness or loose plug
• Low Faulty driver in the engine control module

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