P28AFCylinder 8 Ion Current Sense Circuit High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P28AF is logged when cylinder 8 delivers an ion signal pinned at or above the upper limit. Possible reasons include the signal conductor shorted to battery voltage or to another live circuit in the harness, internal leakage inside coil 8 that lets the high-voltage side couple into the measuring line, or a lost reference ground at the evaluation input. CARB requires input components to be checked for values beyond their valid limits, and a reading stuck high would otherwise feed false combustion and knock data to the engine computer. How close high voltages sit to the signal is visible in Saab's T7 discharge modules, which fired from a 400 V capacitor while holding 80 V on the plug for measurement. No manufacturer documents P28AF publicly. The low reading is P28AE.
- What you may notice
- MIL lit with a high ion reading for cylinder 8
- Misfire on cylinder 8 may stay undetected
- Safety timing reduces power somewhat
- Likely causes
- LikelyCylinder 8 signal conductor shorted to battery voltage
- PossibleInternal leakage in coil 8 coupling into the measuring line
- RareLost reference ground at the evaluation input
- 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: €40 – €650
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.