P21D2: Cylinder 4 Injector B Circuit/Open

What does P21D2 mean?

P21D2 reports an open in the drive circuit of the B injector on cylinder 4. On four-cylinder engines with dual injection it is the last of the set, for example Ford's 2.0 EcoBoost from 2022 or the 2.3 introduced for the 2024 Mustang, both of which added port injectors to direct injection. Ford tests it like its siblings, continuously with at least 11 V and a 5 s window, and assigns letter A to the direct injector (P0204). Mitsubishi, whose Eclipse Cross uses B for the direct injectors, has no open code here and files such faults under the low code P21E2. On Ford's 3.5 V6 cylinder 4 opens the second bank. In GM's 2019 LT5 module the check samples every 100 ms. Expect a misfire on cylinder 4.

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

Symptoms

• Check engine light on
• Misfire on cylinder 4 (P0304)
• Uneven idle

Common causes

• High Open winding in the cylinder 4 port injector
• Medium Wire broken at a bend or clip near the intake manifold
• Medium Corroded or loose injector connector

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