P28A4Cylinder 6 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
- P28A4 is the general circuit code for the ion measurement of cylinder 6: the electronics get no usable signal from that plug. Only engines with six or more cylinders are affected, and among them ion sensing is rare. The V6 of Saab's 9-5 with Trionic T7 used it; there cylinder 6 is in bank 2 with 2 and 4, is triggered from engine module pin 32, and shares its start spark with cylinder 3. The V6 of the 2007 Saab 9-3 is a counterexample: its Motronic ME9 relies on two piezo knock sensors, one per bank, so no ion signal exists there at all. No manufacturer publicly documents P28A4; on an affected engine, coil 6, its wiring and the plug are checked first.
- What you may notice
- MIL with ion fault stored for cylinder 6
- Reduced misfire and knock monitoring on cylinder 6
- Moderate power loss from cautious timing
- Likely causes
- LikelyIon measuring part of the cylinder 6 coil defective
- PossibleOpen ion signal circuit of cylinder 6 in the harness or connector
- RareProcessing input for cylinder 6 failed
- 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 – €650
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.