P28AA code meaning
Cylinder 7 Ion Current Sense Circuit Low
Powertrain system Generic code P28xx codes Check engine light
What P28AA means
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 trunk, 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.
P28AA 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 P28AA
- 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
Common causes of P28AA
- Most likelyCarbon track inside the cylinder 7 plug trunk
- PossibleIon signal lead of cylinder 7 rubbed through to ground
- PossibleSplit ceramic insulator on the cylinder 7 plug
- Less likelyMeasuring-voltage stage in coil 7 no longer working
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Spark plug cyl 7Ignition coil cyl 7Ion sense module
What each part of P28AA 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. |
| AA | The exact fault in that area. |
What to do next
- Note P28AA 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 P28AA
What does P28AA mean?
P28AA means "Cylinder 7 Ion Current Sense Circuit Low". P28AA reports that the ion current reading from cylinder 7 has dropped below the floor the engine computer accepts. It is a generic code, so it means the same on every make.
Is it safe to drive with P28AA?
Drive gently and only to a workshop. With P28AA the car can go into limp mode (reduced power) to protect itself.
Will P28AA fail an emissions test?
Yes, it can. P28AA 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 P28AA 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 P28AA clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.