What it means
The control module detected a low electrical signal in the cylinder 9 injector B circuit. This indicates an electrical circuit condition; it does not by itself establish injector flow or cylinder fuel balance.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Cylinder 9 injector B is the injector identified for cylinder 9, and its driver circuit carries the electrical command used to operate it. The control module monitors the circuit response and sets P21EC when the signal is lower than expected. This identifies a low electrical signal in the cylinder 9 injector B circuit, not a confirmed injector-flow or cylinder-balance fault.
Common causes
Most likely first.
- Short to ground in wiring
- Damaged or corroded 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 check live misfire counts and fuel trims to see when the cylinder 9 injector B circuit fault occurs and whether cylinder 9 is misfiring.
- With the engine off, inspect the cylinder 9 injector B connector and harness for corrosion, loose terminals, chafing, or insulation damage that could let a wire contact ground.
- Use the wiring diagram to check the cylinder 9 injector B circuit for a short to ground, an open, or a poor connection.
- With the circuit isolated as specified by the manufacturer, test the injector’s electrical condition against the manufacturer's specification.
- If the injector and wiring pass, test the cylinder 9 injector B driver circuit at the control module following the manufacturer's procedure.
Common fixes
- Clean and secure a corroded or loose cylinder 9 injector B connector.
- Repair a short to ground, open, or damaged wire in the cylinder 9 injector B circuit.
- Replace cylinder 9 injector B if its electrical test is outside the manufacturer's specification.
- Repair or replace the control module only if its cylinder 9 injector B driver fails the specified tests.
Can I keep driving?
If the engine is smooth and the warning light is steady, keep driving to a minimum and avoid heavy acceleration until the circuit is checked. Stop if the light flashes, the engine misfires severely, or power drops sharply, because misfire can damage the catalytic converter.
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.