P21D2: Cylinder 4 Injector B Circuit/Open

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.

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

Symptoms

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

Most likely causes

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

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: medium
It affects emissions.

What each character of P21D2 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
1: Air and fuel
D2: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21D2?

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 P21D2?

The most common causes are: open winding in the cylinder 4 port injector; wire broken at a bend or clip near the intake manifold; corroded or loose injector connector.

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.