P28B1: Cylinder 9 Ion Current Sense Circuit Range/Performance

What it means

The ion current signal from cylinder 9 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-current system evaluates combustion in cylinder 9 by sensing electrical activity through its spark plug after the spark. This code sets when the cylinder 9 signal is present but outside the expected range or performance pattern, making the ECM's combustion assessment unreliable.

Common causes

Most likely first.

1. Spark plug aged - electrode shape changed
2. Ion sense circuit gain drift
3. Sensor element aged beyond calibration window

Symptoms

• Check-engine light
• Engine misfire or rough running
• Reduced power

How to diagnose it

1. Save freeze-frame data and compare live cylinder 9 ion-current and misfire readings with other cylinders at similar engine conditions.
2. Inspect the cylinder 9 spark plug for electrode wear, deposits, oil fouling, or damage that could alter its sensing response.
3. Check the cylinder 9 sense connector and wiring for corrosion, loose terminals, poor contact, or physical damage.
4. Check for ignition or combustion problems specific to cylinder 9 that could put its ion-current signal outside the normal pattern.
5. If the plug and wiring are sound, test the cylinder 9 sense circuit and module response according to the wiring diagram and manufacturer's specification.

Common fixes

• Replace a worn or fouled cylinder 9 spark plug with the specified type.
• Clean and secure poor cylinder 9 sense-circuit connector contacts.
• Repair damaged cylinder 9 sense wiring or terminals.
• Replace or recalibrate a faulty ion-sense component only when testing confirms gain drift or an out-of-calibration sensor.

Can I keep driving?

If the engine runs normally and the warning light is steady, drive gently and arrange diagnosis soon because cylinder 9 combustion monitoring may be unreliable and emissions may rise. Stop if it begins to misfire or loses power.