P289F: Cylinder 4 Ion Current Sense Circuit High

What it means

Ion current from cylinder 4 is above the expected window. Cross-coupled signal, biased reference, or sensor fault.

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 current in combustion gases after the spark, helping the ECM evaluate that cylinder's burn. This code sets when the cylinder 4 signal is higher than expected, which can result from ignition-coil interference, a biased reference, or a poor connector contact.

Common causes

Most likely first.

1. Inductive cross-coupling from ignition coil primary
2. Sense reference voltage biased
3. Connector pins corroded - high contact resistance

Symptoms

• Check-engine light on
• Rough idle or engine misfire
• Reduced engine performance

How to diagnose it

1. Save freeze-frame data and review live cylinder 4 ion-sense and misfire readings to find when the signal becomes too high.
2. Inspect the cylinder 4 plug, ion-sense connector, and harness for corrosion, loose pins, damaged insulation, or wiring routed close to ignition-coil wiring.
3. Check connector terminal fit and sense-wire continuity for excessive contact resistance or an unintended connection to another circuit.
4. Measure the cylinder 4 ion-sense reference and signal circuits during the fault and compare readings with the manufacturer's specification.
5. If wiring and reference checks pass, investigate coil-primary interference and test the ion-sense module with the manufacturer's procedure.

Common fixes

• Clean and secure the cylinder 4 ion-sense connector or repair corroded terminals.
• Repair damaged sense wiring and restore the specified separation from ignition-coil wiring.
• Correct a biased ion-sense reference circuit fault.
• Replace a confirmed faulty ion-sense module or sensing component.

Can I keep driving?

If the engine runs smoothly, a short, gentle trip to diagnosis may be reasonable; avoid hard acceleration and heavy loads. Stop driving if a strong misfire develops, as continued misfiring can damage the catalytic converter.