What it means
The control module detected an open or otherwise faulty electrical circuit for cylinder 4 injector B. The code identifies the circuit fault; it does not establish what injector B refers to on a particular engine.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The engine controller operates injector B on cylinder 4 and monitors its electrical circuit for the expected response. P21D2 sets when the cylinder 4 injector B circuit is detected as open or otherwise electrically faulty; injector B's meaning depends on the engine's design.
Common causes
Most likely first.
- Open or damaged wiring
- Loose or corroded injector connector
- Injector electrical fault
Symptoms
- Check-engine light
- Rough idle or engine vibration
- Engine hesitation or reduced power
- Misfire, especially under load
How to diagnose it
- Save freeze-frame data and check cylinder-specific misfire data and related injector codes to determine when cylinder 4 was affected.
- Locate cylinder 4 and injector B using vehicle service information, then inspect its harness and connector for disconnection, corrosion, chafing, or heat damage.
- Check the cylinder 4 injector B connector terminals for bent, loose, spread, or pushed-back pins.
- Test the injector B circuit for continuity and shorts using the wiring diagram and manufacturer's diagnostic procedure.
- Check the injector's electrical operation against the manufacturer's specification to confirm whether the injector or circuit is at fault.
Common fixes
- Reconnect and secure the cylinder 4 injector B connector.
- Repair an open or damaged wire in the cylinder 4 injector B circuit.
- Clean or repair corroded or damaged connector terminals.
- Replace cylinder 4 injector B if testing confirms an electrical fault.
Can I keep driving?
If the engine is misfiring or shaking, avoid driving under load because continued misfire can damage the catalytic converter and reduce power. If it runs smoothly, keep trips short and repair the cylinder 4 injector B circuit soon.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.