What it means
The ion current signal from cylinder 4 is outside the expected range for combustion conditions. Combustion quality cannot be assessed reliably.
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 reads current at the cylinder 4 spark plug after combustion so the ECM can assess that cylinder's burn. This code sets when the cylinder 4 signal is present but its range or response falls outside the expected pattern, possibly because the plug is worn, circuit gain has drifted, or the sensing element has aged.
Common causes
Most likely first.
- Spark plug aged - electrode shape changed
- Ion sense circuit gain drift
- Sensor element aged beyond calibration window
Symptoms
- Check-engine light on
- Rough idle or engine misfire
- Reduced engine performance
How to diagnose it
- Capture freeze-frame data and compare live cylinder 4 ion-sense and misfire readings at idle and under the conditions that set the code.
- Inspect the cylinder 4 spark plug for wear or a changed electrode shape and assess it against the manufacturer's service guidance.
- Check the cylinder 4 ion-sense connector and harness for corrosion, loose pins, insulation damage, or routing problems.
- Test the signal, reference, and ground circuits for continuity and stable readings against the manufacturer's specification.
- If the plug and wiring pass inspection, follow the service procedure to assess ion-sense gain and sensing-element drift.
Common fixes
- Replace a worn cylinder 4 spark plug with the specified type and gap.
- Clean and secure the cylinder 4 ion-sense connector or repair damaged terminals.
- Repair circuit faults affecting the signal, reference, or ground.
- Replace a confirmed degraded ion-sense module or sensing element.
Can I keep driving?
If the engine runs smoothly, a short, gentle trip for diagnosis may be reasonable; avoid hard acceleration and heavy loads. Persistent roughness or misfire can worsen and may 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.