P28A1Cylinder 5 Ion Current Sense Circuit Range/Performance
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- P28A1 flags a cylinder 5 ion signal that is electrically intact but implausible in size or shape compared with load, speed and the other cylinders. Ion current rises with chamber pressure and temperature, so a lean or poorly compressing cylinder 5 produces a weak trace exactly like a fouled plug does. Saab's T7 V6 shows how closely the signal is tied to misfire detection: its engine module accumulates missing combustion signals per cylinder over 1000 revolutions and compares them with a load- and speed-dependent limit before setting P0305. That discharge module also evaluates knock from the ion flow of its bank. No manufacturer documents P28A1 publicly; electrical faults on cylinder 5 are P28A0, P28A2 and P28A3.
- What you may notice
- MIL, possibly with misfire code P0305
- Uneven running under load
- Higher fuel consumption
- Likely causes
- LikelyFouled or eroded spark plug in cylinder 5
- PossibleLean mixture in cylinder 5 from a clogged injector or intake leak
- RareLow compression in cylinder 5
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Easy · Often possible to do yourself.Typical cost: €30 – €450
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.