What it means
The ECM cannot read a valid ion current signal from cylinder 4. Ion current sensing relies on the spark plug acting as an electrode after combustion.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The ion-sensing system uses the spark plug in cylinder 4 as an electrode to detect current in combustion gases after the spark, so the ECM can assess the burn. This code sets when the ECM cannot obtain a valid cylinder 4 ion-current signal, often because the ion-sense module or its wiring and connector cannot provide a usable reading.
Common causes
Most likely first.
- Ion sense module failed
- Wiring or connector fault
Symptoms
- Check-engine light on
- Rough idle or engine misfire
- Reduced engine performance
How to diagnose it
- Save freeze-frame data and review live cylinder 4 misfire and ion-sense information to identify when the signal becomes invalid.
- Inspect the cylinder 4 spark plug area, ion-sense connector, and harness for loose connections, corrosion, oil, damaged insulation, or disturbed wiring.
- Check cylinder 4 sense-wire continuity and connector terminal fit using the wiring diagram, looking for an open, short, or high-resistance contact.
- Check module power, ground, reference, and signal circuits under the conditions specified by the manufacturer.
- If wiring and circuit supplies meet specification, test the ion-sense module using the manufacturer's procedure before replacing it.
Common fixes
- Reconnect or clean the cylinder 4 ion-sense connector and repair corroded terminals.
- Repair an open, shorted, or damaged section of cylinder 4 sense wiring.
- Restore a faulty module power, ground, or reference connection.
- Replace the ion-sense module if circuit tests confirm it has failed.
Can I keep driving?
If the engine runs smoothly, limit driving to a short trip for diagnosis and avoid heavy loads. If it misfires or loses power, stop driving when safe because unburned fuel can overheat and 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.