What it means
The control module detected a low electrical signal in the cylinder 1 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
The engine control module monitors the electrical signal in the circuit for cylinder 1 injector B. P21DB sets when that signal is lower than expected, often pointing to a short to ground or a poor connection; it describes circuit voltage, not injector flow or cylinder fuel balance.
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 review cylinder 1 misfire and injector codes to see when the low circuit signal occurs.
- Use service information to locate cylinder 1 injector B and inspect its connector and harness for rubbed insulation, corrosion, moisture, or a loose terminal.
- With ignition off, check the cylinder 1 injector B connector terminals and wiring near the injector for a path to ground caused by damaged insulation.
- Follow the manufacturer's procedure to measure the cylinder 1 injector B circuit signal and check for an unintended short to ground, comparing results with specification.
- If the harness and connections are sound, test injector B on cylinder 1 electrically to determine whether an internal fault is pulling the circuit low.
Common fixes
- Clean and secure the cylinder 1 injector B connector or repair a corroded terminal.
- Repair wiring insulation or a short to ground in the cylinder 1 injector B circuit.
- Replace cylinder 1 injector B if it fails the specified electrical test.
Can I keep driving?
If the engine runs normally and the light is steady, limit driving and avoid hard acceleration until the low signal is diagnosed. Stop if the light flashes or cylinder 1 misfires badly, since continued misfire can worsen engine and emissions-system damage.
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.