P28BE: Cylinder 12 Ion Current Sense Circuit Low

What it means

Ion current from cylinder 12 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 spark plug in cylinder 12 to detect post-spark electrical activity that indicates combustion. The engine control module sets P28BE when cylinder 12's ion-current signal falls below its expected window, which can result from a fouled plug or a signal path pulled low or interrupted.

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
• Engine misfire or rough running
• Reduced power

How to diagnose it

1. Save freeze-frame data and review cylinder 12 misfire counts and ion-sense live data to confirm when the signal drops low.
2. Remove and inspect cylinder 12's spark plug for carbon or oil fouling that can weaken the sensed signal.
3. Inspect the cylinder 12 sense wire and connector pins for a short to ground, corrosion, poor contact, or damage.
4. Check continuity and shorts to ground in the cylinder 12 sense circuit using the wiring diagram and manufacturer's specification.
5. If wiring and plug checks pass, test the ion-sense module input for cylinder 12 according to service information.

Common fixes

• Replace a carbon- or oil-fouled cylinder 12 spark plug after addressing any source of contamination.
• Clean and secure corroded or loose cylinder 12 connector pins.
• Repair a cylinder 12 sense wire shorted to ground or an open in the sensing path.
• Replace the ion-sense module if circuit testing confirms its cylinder 12 input is faulty.

Can I keep driving?

A steady warning light with normal running may allow a short, gentle trip for diagnosis, but avoid heavy loads because a weak cylinder 12 signal can accompany a misfire and raise emissions. Stop if the engine runs very rough, loses power, or the light flashes, since continued misfiring can damage the exhaust catalyst.