P21D4: Cylinder 6 Injector B Circuit/Open

P21D4 is the last B injector code on a six-cylinder engine. Ford's second-generation 2.7 and 3.5 EcoBoost V6 combine port and direct injection, label the port injectors B and test this circuit together with P21CF to P21D3, continuously above 11 V within 5 s; on these engines cylinder 6 is the rear cylinder of the second bank. On the 3.5 the direct system is shut down at low speed and light load and the engine runs on port fuel alone, so an open cylinder 6 port injector is most noticeable at idle and in gentle driving. GM's 2019 LT5 V8 also covers cylinder 6 in its injection module, Type A after 20 of 25 failed samples. The A-side direct injector code is P0206, the misfire code P0306; P21E6 and P21E7 are the low and high faults.

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

Symptoms

Check engine light on
Misfire on cylinder 6 (P0306)
Slightly rough idle

Most likely causes

Port injector of cylinder 6 open internally
Damaged wire at the rear of the engine, hard to inspect
Connector not fully latched after earlier work on the intake

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 P21D4 means

P: Powertrain: engine and transmission
2: Generic code: same on every make
1: Air and fuel
D4: Specific fault in that system

Frequently asked questions

Can I keep driving with code P21D4?

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 P21D4?

The most common causes are: port injector of cylinder 6 open internally; damaged wire at the rear of the engine, hard to inspect; connector not fully latched after earlier work on the intake.

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.