P28AF code meaning
Cylinder 8 Ion Current Sense Circuit High
Powertrain system Generic code P28xx codes Check engine light
What P28AF means
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.
P28AF at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium, DIY possible
- Typical labor0.3 to 2.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P28AF
- MIL lit with a high ion reading for cylinder 8
- Misfire on cylinder 8 may stay undetected
- Safety timing reduces power somewhat
Common causes of P28AF
- Most likelyCylinder 8 signal conductor shorted to battery voltage
- PossibleInternal leakage in coil 8 coupling into the measuring line
- Less likelyLost reference ground at the evaluation input
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 P28AF 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. |
| AF | The exact fault in that area. |
What to do next
- Note P28AF 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 P28AF
What does P28AF mean?
P28AF means "Cylinder 8 Ion Current Sense Circuit High". P28AF is logged when cylinder 8 delivers an ion signal pinned at or above the upper limit. It is a generic code, so it means the same on every make.
Is it safe to drive with P28AF?
Drive gently and only to a workshop. With P28AF the car can go into limp mode (reduced power) to protect itself.
Will P28AF fail an emissions test?
Yes, it can. P28AF 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 P28AF to fix?
Difficulty: medium. A typical repair takes 0.3 to 2.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P28AF clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.