What it means
The control module detected an open or otherwise faulty electrical circuit for cylinder 12 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 used to actuate injector B on cylinder 12. P21DA is set when the circuit is detected as open or electrically faulty, potentially disrupting cylinder 12 fueling; the code alone does not tell you how injector B is implemented on 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
- Retrieve freeze-frame data and check for cylinder 12 misfire or injector codes to establish the operating conditions when the fault set.
- Identify cylinder 12 injector B from service information, then inspect its connector and harness for damage, corrosion, poor routing, or a loose connection.
- With the ignition off, inspect the cylinder 12 injector B plug for contaminated, bent, or recessed terminals and refit it securely.
- Test continuity in the cylinder 12 injector B circuit and compare with the manufacturer's specification to check for an open wire.
- If wiring checks pass, electrically test injector B at cylinder 12 for an internal circuit fault.
Common fixes
- Secure the cylinder 12 injector B connector and clean or repair its terminals.
- Repair an open or damaged section of cylinder 12 injector B wiring.
- Replace cylinder 12 injector B if it fails electrical testing.
Can I keep driving?
A steady light with normal running calls for limited, gentle driving until diagnosis, because a cylinder 12 fueling fault can increase emissions and fuel use. Avoid heavy loads and stop if the light flashes or the engine develops a strong misfire.
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.