P289A: Cylinder 3 Ion Current Sense Circuit Low

What it means

Ion current from cylinder 3 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 3 spark plug to detect electrical current in the combustion gases after the spark. This code sets when the cylinder 3 signal falls below the expected range, which can occur with a carbon- or oil-fouled plug, a sense wire shorted to ground, or a poor connector contact.

Common causes

Most likely first.

1. Spark plug fouled with carbon or oil
2. Sense wire shorted to ground
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. Record freeze-frame data and inspect live cylinder 3 ion-sense and misfire readings to establish when the low signal occurs.
2. Remove and inspect the cylinder 3 spark plug for carbon or oil fouling, and check whether the plug condition points to an underlying engine issue.
3. Inspect the cylinder 3 sense wire and connector for a short to ground, chafing, corrosion, loose pins, or contamination.
4. Check sense-wire continuity and isolation from ground with the circuit handled as specified by the manufacturer.
5. If the plug and wiring pass, measure the sense reference and signal path and test the ion-sense module against the manufacturer's procedure.

Common fixes

• Replace or service a fouled cylinder 3 spark plug and address the source of oil or carbon deposits.
• Clean and secure the cylinder 3 connector or repair corroded terminals.
• Repair the cylinder 3 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 send unburned fuel into the exhaust, potentially damaging the catalytic converter.