P28AACylinder 7 Ion Current Sense Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P28AA reports that the ion current reading from cylinder 7 has dropped below the floor the engine computer accepts. Ion sensing places a measuring voltage on the spark plug between firings; Saab's Trionic T8 coils generate about 120 V for this themselves. If that voltage leaks away through a carbon track inside the plug boot, a split insulator or a signal lead rubbing on the block, the evaluator sees almost no current even when the mixture burns normally. Because the reading then resembles a dead cylinder, a misfire code such as P0307 may be stored alongside. Cylinder 7 exists only on V8 and larger engines, and no maker publicly documents an ion-sensing engine of that size or this code. P28AB covers the opposite fault, P28A8 the open circuit.
- What you may notice
- MIL lit with a low ion reading stored for cylinder 7
- Misfire code P0307 may appear although the engine runs smoothly
- Reduced power from protective ignition timing
- Likely causes
- LikelyCarbon track inside the cylinder 7 plug boot
- PossibleIon signal lead of cylinder 7 rubbed through to ground
- PossibleSplit ceramic insulator on the cylinder 7 plug
- RareMeasuring-voltage stage in coil 7 no longer working
- 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: €30 – €600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.