What it means
The control module detected an open or otherwise faulty electrical circuit for cylinder 9 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 checks the electrical path for injector B on cylinder 9. It sets P21D7 when that path is detected as open or electrically faulty, which can interrupt commanded fueling to cylinder 9; the code does not establish what the B designation refers to for this 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 look for cylinder 9 misfire or injector-related codes that may help confirm the fault conditions.
- Locate cylinder 9 injector B from service information and inspect its connector and harness for corrosion, loose fit, broken insulation, or chafing.
- With ignition off, check that the connector terminals at cylinder 9 injector B are clean, seated, and making firm contact.
- Test wiring continuity along the cylinder 9 injector B circuit and compare readings with the manufacturer's specification.
- If the connector and wiring pass, test injector B at cylinder 9 electrically to determine whether the injector itself is faulty.
Common fixes
- Reconnect the cylinder 9 injector B plug and clean or repair poor terminal contact.
- Repair damaged or open wiring serving cylinder 9 injector B.
- Replace cylinder 9 injector B if it fails the manufacturer's electrical test.
Can I keep driving?
A steady warning light without rough running may allow a short, gentle drive for diagnosis, but the fault can cause a cylinder 9 misfire and increased emissions. Stop if the light flashes or the engine runs very unevenly, and avoid high loads until repaired.
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.