P28A9 code meaning
Cylinder 7 Ion Current Sense Circuit Range/Performance
Powertrain system Generic code P28xx codes Check engine light
What P28A9 means
P28A9 means cylinder 7 returns an ion signal that is electrically sound but not believable for the current speed and load, or that stands out against the other cylinders of an eight-or-more-cylinder engine. Because the current through the plug gap grows with combustion pressure and temperature, a lean or misfiring cylinder 7 produces this picture as readily as a fouled plug or moisture in the well. The signal serves both misfire detection and knock control, as Saab's ion-sensing T7 V6 illustrates by analysing each bank's ion flow for knock. No manufacturer documents P28A9 publicly, and no production V8 with this measurement is publicly described. Electrical faults on cylinder 7 are reported as P28A8, P28AA and P28AB.
P28A9 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 2.5 hours
- Clears after repairAfter a drive cycle
Symptoms of P28A9
- MIL, possibly together with P0307
- Uneven idle
- Knock control retards timing more often
Common causes of P28A9
- Most likelyFouled or worn spark plug in cylinder 7
- PossibleLean mixture or misfire in cylinder 7
- Less likelyMoisture in the plug well of cylinder 7
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 P28A9 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. |
| A9 | The exact fault in that area. |
What to do next
- Note P28A9 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 P28A9
What does P28A9 mean?
P28A9 means "Cylinder 7 Ion Current Sense Circuit Range/Performance". P28A9 means cylinder 7 returns an ion signal that is electrically sound but not believable for the current speed and load, or that stands out against the other cylinders of an eight-or-more-cylinder engine. It is a generic code, so it means the same on every make.
Is it safe to drive with P28A9?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P28A9 fail an emissions test?
Yes, it can. P28A9 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 P28A9 to fix?
Difficulty: easy. 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 P28A9 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.