P28AC code meaning
Cylinder 8 Ion Current Sense Circuit
Powertrain system Generic code P28xx codes Check engine light
What P28AC means
P28AC flags a general electrical fault in the ion current path of cylinder 8, typically an open connection, so no usable signal reaches the evaluation electronics. On a V8 this is the highest-numbered cylinder, on V10 and V12 engines a middle one; which bank it sits on depends on the maker's numbering. A cylinder-specific code presumes one input per coil, as in the CDM of Saab's later Trionic T8, which receives a separate ion line from each of the four coils. Older T7 modules merged cylinders into shared detectors and could not name a single one. No manufacturer documents P28AC, and no eight-cylinder ion-sensing engine is publicly known. Check coil 8, its plug, connector and wiring first; range and level faults are P28AD, P28AE and P28AF.
P28AC at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium, DIY possible
- Typical labor0.3 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P28AC
- MIL with a circuit fault stored for cylinder 8
- No misfire or knock assessment for cylinder 8
- Engine may run with conservative timing and lower power
Common causes of P28AC
- Most likelyLoose or corroded connector in the cylinder 8 ion line
- PossibleMeasuring stage of coil 8 failed
- PossibleBroken conductor between coil 8 and the evaluation module
- Less likelyCylinder 8 input of the evaluation module defective
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Spark plug cyl 8Ignition coil cyl 8Ion sense module
What each part of P28AC means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 8 | Area: transmission. |
| AC | The exact fault in that area. |
What to do next
- Note P28AC and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P28AC
What does P28AC mean?
P28AC means "Cylinder 8 Ion Current Sense Circuit". P28AC flags a general electrical fault in the ion current path of cylinder 8, typically an open connection, so no usable signal reaches the evaluation electronics. It is a generic code, so it means the same on every make.
Is it safe to drive with P28AC?
Drive gently and only to a workshop. With P28AC the car can go into limp mode (reduced power) to protect itself.
Will P28AC fail an emissions test?
Yes, it can. P28AC is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P28AC to fix?
Difficulty: medium. A typical repair takes 0.3 to 3 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P28AC clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.