P21F3: Cylinder 12 Injector B Circuit High

What does P21F3 mean?

P21F3 ends the B injector series of SAE J2012: a high signal on the cylinder 12 circuit, caused by a short to supply on the switched side. The series runs from P21CF to P21DA for opens and from P21DB to P21F3 for low/high pairs; the A injector equivalent here is P0295. Public manufacturer documents only reach cylinder 8 (GM's LT5 V8) or cylinder 4 (Mitsubishi), and no twelve-cylinder with dual injection could be documented, since Lamborghini's and Aston Martin's V12s use port injection alone. Should the code appear, the injector cannot open; cylinder 12 then relies on its other injector, and the driver stage takes the brunt of each switching attempt. GM's criteria for the same fault type serve as a guide: within 0.5 Ohm of power with the driver off. Low fault: P21F2.

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

Symptoms

• Check engine light on
• Cylinder 12 lean or misfiring (P0312)

Common causes

• High Control wire of the cylinder 12 B injector shorted to supply
• Medium Injector coil with shorted windings
• Low Driver stage damaged by a lasting short

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