P034C
Ion Sense System Performance
Some engines (notably Saab and some Mazda direct-injection units) use the spark plug as an ion-current sensor after spark to detect knock and misfire. This code reports a performance issue with the ion-sense input — typically a worn plug, loose secondary connection, or aged ion-sense module.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: hard
Common causes
Ordered by how often each turns out to be the cause.
- Spark plugs wornCommon
- Ion-sense module agedPossible
Parts involved
- Ion Sense Module
- Spark Plug
- Ignition Coil
P034C is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.