What it means
Ion current from cylinder 4 is below the expected window. Spark plug fouled, signal wire shorted to ground, or sensing path interrupted.
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 cylinder 4 spark plug to detect electrical current in the combustion gases after the spark. This code sets when the cylinder 4 signal drops below its expected range, which may be caused by a carbon- or oil-fouled plug, a sense wire short to ground, or a poor connector contact.
Common causes
Most likely first.
- Spark plug fouled with carbon or oil
- Sense wire shorted to ground
- Connector pins corroded - high contact resistance
Symptoms
- Check-engine light on
- Rough idle or engine misfire
- Reduced engine performance
How to diagnose it
- Record freeze-frame data and check live cylinder 4 ion-sense and misfire readings to determine when the low signal appears.
- Inspect the cylinder 4 spark plug for carbon or oil fouling and consider what the plug condition indicates about the cause.
- Inspect the cylinder 4 sense wire and connector for a ground short, chafing, corrosion, loose pins, or contamination.
- Check sense-wire continuity and isolation from ground according to the manufacturer's circuit procedure.
- If the plug and wiring pass, measure the reference and signal path and assess the ion-sense module using the manufacturer's procedure.
Common fixes
- Replace or service a fouled cylinder 4 spark plug and correct the source of oil or carbon deposits.
- Clean and secure the cylinder 4 connector or repair corroded terminals.
- Repair the cylinder 4 sense wire where it is shorted to ground or damaged.
- Replace a confirmed faulty ion-sense module or sensing element.
Can I keep driving?
If the engine is smooth, keep any necessary drive short and gentle, and avoid heavy loads. A rough misfire can worsen and allow unburned fuel into the exhaust, potentially damaging 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.