P289DCylinder 4 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
- P289D is a rationality fault: cylinder 4 supplies an ion trace that is electrically present but not credible, for instance a combustion peak far smaller than the others at equal load, or erratic changes between cycles. Low-pass filtering of this trace gives the combustion information, band-pass filtering the knock content, so a distorted signal degrades both functions for cylinder 4. On the 2009 Saab module, the cylinder 4 ion input is pin 15 and it is combined with cylinder 2 into the second combustion signal. Wear or fouling of the plug, an oil-wetted plug well, or a genuine mixture or compression problem in cylinder 4 are typical triggers. No manufacturer publicly documents P289D; P289C, P289E and P289F cover the electrical faults.
- What you may notice
- MIL, sometimes accompanied by P0304
- Noticeable vibration at idle
- Fuel consumption rises slightly
- Likely causes
- LikelyWorn or fouled spark plug in cylinder 4
- PossibleOil in the plug well of cylinder 4 from a leaking cover gasket
- PossibleMixture or compression problem in cylinder 4
- 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.