What it means
The control module detected an open or otherwise faulty electrical circuit for cylinder 6 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 control module monitors the electrical path that operates injector B for cylinder 6. P21D4 is set when the circuit is detected as open or electrically faulty, which can prevent the commanded fuel delivery to cylinder 6; the code does not define injector B’s design on a particular engine.
Common causes
Most likely first.
- Open or damaged wiring
- Loose or corroded injector connector
- Injector electrical fault
Symptoms
- Check-engine light on
- Rough running or misfire
- Reduced engine power
- Increased fuel consumption
How to diagnose it
- Save freeze-frame data and scan for cylinder 6 misfire or injector codes to establish the conditions present when the fault was recorded.
- Confirm cylinder 6 and injector B locations from vehicle service information, then inspect that injector’s connector and harness for damage, corrosion, or a loose fit.
- With ignition off, check the cylinder 6 injector B connector terminals for contamination, poor contact, or a terminal pushed out of position.
- Test the cylinder 6 injector B wiring for continuity and unintended opens, comparing readings with the manufacturer's specification.
- If the circuit checks out, test the cylinder 6 injector B electrically to see whether the injector has an internal fault.
Common fixes
- Reconnect the cylinder 6 injector B plug and clean or repair poor terminal contact.
- Repair an open, chafed, or damaged section of the cylinder 6 injector B wiring.
- Replace the cylinder 6 injector B if it fails the specified electrical test.
Can I keep driving?
A steady warning light with normal engine operation usually permits only limited driving to a repair location, but the cylinder 6 fault can cause rough running, extra fuel use, and higher emissions. Avoid heavy loads and stop if the light flashes or the engine begins misfiring severely.
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.