P28A4: Cylinder 6 Ion Current Sense Circuit

What it means

The ECM cannot read a valid ion current signal from cylinder 6. 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 spark plug at cylinder 6 acts as an electrode after ignition, allowing the ECM's ion-sense system to read combustion activity. This code sets when the ECM cannot obtain a valid ion-current signal from cylinder 6, pointing to its sensing module, wiring, or connector.

Common causes

Most likely first.

1. Ion sense module failed
2. Wiring or connector fault

Symptoms

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

How to diagnose it

1. Read freeze-frame data and live cylinder-specific misfire or ion-sense information to confirm the fault is associated with cylinder 6.
2. Inspect the cylinder 6 ion-sense connector and harness for loose connections, corrosion, chafing, or heat damage.
3. Check that the cylinder 6 spark plug and its connection are intact, since the plug is part of the ion-current sensing path.
4. Test the cylinder 6 sense wiring for continuity and shorts using the manufacturer's circuit information and specification.
5. If the wiring and connections pass, test the ion-sense module or sensing element for cylinder 6 according to the manufacturer's procedure.

Common fixes

• Reconnect and secure a loose cylinder 6 ion-sense connector.
• Clean corroded connector contacts or replace damaged terminals.
• Repair chafed, broken, or shorted cylinder 6 sense wiring.
• Replace the failed ion-sense module or sensing element if circuit checks confirm an internal fault.

Can I keep driving?

If the engine is running smoothly, limit driving until the cylinder 6 signal is restored because combustion monitoring may be unreliable and emissions may increase. Stop if it misfires severely or loses substantial power.