P28A8Cylinder 7 Ion Current Sense Circuit
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P28A8 covers a basic circuit fault in the ion current measurement of cylinder 7, so the module obtains no evaluable signal from that plug. A cylinder 7 only exists on engines with eight or more cylinders, and Saab, the best-known user of ion sensing, applied it to four-cylinder and V6 engines; no maker publicly describes an eight-cylinder version. No manufacturer publicly documents P28A8. Its physical position depends on the maker's numbering, which differs between brands on V8 engines. The principle stays the same: a bias voltage across the plug gap lets the burning gas conduct a small current, and a lost signal blinds misfire and knock assessment for that cylinder. Diagnosis starts at coil 7, its connector and the spark plug.
- What you may notice
- MIL with ion fault stored for cylinder 7
- Misfire and knock monitoring of cylinder 7 unavailable
- Some power loss from protective timing
- Likely causes
- LikelyCoil of cylinder 7 with defective ion measuring function
- PossibleBroken wire or loose connector in the cylinder 7 ion circuit
- RareCylinder 7 input of the evaluation electronics faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €60 – €700
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.