P28A4 code meaning
Cylinder 6 Ion Current Sense Circuit
Powertrain system Generic code P28xx codes Check engine light
What P28A4 means
P28A4 is the general circuit code for the ion measurement of cylinder 6: the electronics get no usable signal from that plug. Only engines with six or more cylinders are affected, and among them ion sensing is rare. The V6 of Saab's 9-5 with Trionic T7 used it; there cylinder 6 is in bank 2 with 2 and 4, is triggered from engine module pin 32, and shares its start spark with cylinder 3. The V6 of the 2007 Saab 9-3 is a counterexample: its Motronic ME9 relies on two piezo knock sensors, one per bank, so no ion signal exists there at all. No manufacturer publicly documents P28A4; on an affected engine, coil 6, its wiring and the plug are checked first.
P28A4 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 P28A4
- MIL with ion fault stored for cylinder 6
- Reduced misfire and knock monitoring on cylinder 6
- Moderate power loss from cautious timing
Common causes of P28A4
- Most likelyIon measuring part of the cylinder 6 coil defective
- PossibleOpen ion signal circuit of cylinder 6 in the harness or connector
- Less likelyProcessing input for cylinder 6 failed
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Spark plug cyl 6Ignition coil cyl 6Ion sense module
What each part of P28A4 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. |
| A4 | The exact fault in that area. |
What to do next
- Note P28A4 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 P28A4
What does P28A4 mean?
P28A4 means "Cylinder 6 Ion Current Sense Circuit". P28A4 is the general circuit code for the ion measurement of cylinder 6: the electronics get no usable signal from that plug. It is a generic code, so it means the same on every make.
Is it safe to drive with P28A4?
Drive gently and only to a workshop. With P28A4 the car can go into limp mode (reduced power) to protect itself.
Will P28A4 fail an emissions test?
Yes, it can. P28A4 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 P28A4 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 P28A4 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.