What it means
The ECM cannot read a valid ion current signal from cylinder 11. 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 ion-current system uses the spark plug in cylinder 11 as an electrode to sense electrical activity after ignition, which helps the engine control module assess combustion. P28B8 sets when the module cannot obtain a valid cylinder 11 signal, pointing to a fault in the sensing module, wiring, connector, or sensing path.
Common causes
Most likely first.
- Ion sense module failed
- Wiring or connector fault
Symptoms
- Check-engine light
- Engine misfire or rough running
- Reduced power
How to diagnose it
- Save freeze-frame data and check cylinder 11 misfire counts and available ion-sense data to see when the signal becomes invalid.
- Inspect cylinder 11's spark plug connection, sense wiring, and connectors for looseness, corrosion, chafing, or heat damage.
- Check whether the cylinder 11 spark plug is damaged or fouled and verify it is correctly seated and connected.
- Test the cylinder 11 sense wiring for opens, shorts, and poor connector contact using the wiring diagram and manufacturer's specification.
- If the wiring and plug path are sound, test the ion-sense module and its cylinder 11 channel according to service information.
Common fixes
- Reconnect or clean a loose or corroded cylinder 11 connector.
- Repair damaged or open wiring in the cylinder 11 ion-sense path.
- Replace a damaged cylinder 11 spark plug with the specified type.
- Replace the ion-sense module if testing confirms its cylinder 11 channel has failed.
Can I keep driving?
With a steady warning light and smooth running, limit driving and arrange diagnosis soon because combustion in cylinder 11 may not be monitored reliably, increasing emissions or hiding a developing misfire. Stop if the engine runs very rough, loses substantial power, or the check-engine light flashes.
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.