What it means
Ion current from cylinder 9 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-current system senses electrical activity through the cylinder 9 spark plug after ignition to assess combustion. This code sets when the cylinder 9 signal is below its expected window, with a fouled plug, a signal wire short to ground, or high-resistance connector contact among the possible causes.
Common causes
Most likely first.
- Spark plug fouled with carbon or oil
- Sense wire shorted to ground
- Connector pins corroded - high contact resistance
Symptoms
- Check-engine light
- Engine misfire or rough running
- Reduced power
How to diagnose it
- Capture freeze-frame data and check live cylinder 9 misfire and ion-current readings to see when the low signal occurs.
- Inspect the cylinder 9 spark plug for carbon or oil fouling and check the ignition connection for damage.
- Look over the cylinder 9 sense wire for chafing against ground and inspect connector pins for corrosion, looseness, or poor contact.
- Use the wiring diagram to test the cylinder 9 signal circuit for a short to ground and verify continuity to the manufacturer's specification.
- If the plug and wiring pass, test the cylinder 9 ion-sense path and module response using the vehicle's specified procedure.
Common fixes
- Replace a carbon-fouled or oil-fouled cylinder 9 spark plug and address any evident source of contamination.
- Clean and secure corroded or loose cylinder 9 sense-circuit terminals.
- Repair the cylinder 9 sense wire where it is damaged or shorted to ground.
- Replace a failed ion-sense component if testing confirms the sensing path is faulty.
Can I keep driving?
A steady warning light with smooth running may allow a short, gentle drive, but poor combustion can raise emissions and worsen into a misfire. Stop if the engine runs roughly or loses power, and avoid heavy loads.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.