What it means
The control module detected an open or otherwise faulty electrical circuit for cylinder 10 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 circuit that operates injector B for cylinder 10. P21D8 is stored when the circuit is detected as open or electrically faulty, potentially preventing cylinder 10 from receiving its commanded fuel; the code does not define injector B’s physical configuration.
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
- Check freeze-frame data and any cylinder 10 misfire or injector codes to see whether the fault is persistent or tied to particular conditions.
- Use service information to find cylinder 10 injector B, then inspect its wiring route and connector for breaks, corrosion, looseness, or contact with hot or moving parts.
- With ignition off, unplug the cylinder 10 injector B connector and inspect for damaged or recessed terminals before reconnecting it.
- Measure continuity in the cylinder 10 injector B circuit and compare with the manufacturer's specification to locate an open in the harness.
- If circuit integrity is confirmed, test injector B on cylinder 10 electrically for an internal fault.
Common fixes
- Reseat the cylinder 10 injector B connector and clean or repair corroded contacts.
- Repair an open or physically damaged section of the cylinder 10 injector B wiring.
- Replace cylinder 10 injector B if it fails electrical testing.
Can I keep driving?
If the light is steady and the engine remains smooth, restrict driving to necessary short trips and arrange diagnosis. Do not continue with a flashing light, severe shake, or major power loss, as ongoing misfire can worsen damage and emissions.
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.