P28A1 code meaning
Cylinder 5 Ion Current Sense Circuit Range/Performance
Powertrain system Generic code P28xx codes Check engine light
What P28A1 means
P28A1 flags a cylinder 5 ion signal that is electrically intact but implausible in size or shape compared with load, speed and the other cylinders. Ion current rises with chamber pressure and temperature, so a lean or poorly compressing cylinder 5 produces a weak trace exactly like a fouled plug does. Saab's T7 V6 shows how closely the signal is tied to misfire detection: its engine module accumulates missing combustion signals per cylinder over 1000 revolutions and compares them with a load- and speed-dependent limit before setting P0305. That discharge module also evaluates knock from the ion flow of its bank. No manufacturer documents P28A1 publicly; electrical faults on cylinder 5 are P28A0, P28A2 and P28A3.
P28A1 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 2 hours
- Clears after repairAfter a drive cycle
Symptoms of P28A1
- MIL, possibly with misfire code P0305
- Uneven running under load
- Higher fuel consumption
Common causes of P28A1
- Most likelyFouled or eroded spark plug in cylinder 5
- PossibleLean mixture in cylinder 5 from a clogged injector or intake leak
- Less likelyLow compression in cylinder 5
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Spark plug cyl 5Ignition coil cyl 5Ion sense module
What each part of P28A1 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. |
| A1 | The exact fault in that area. |
What to do next
- Note P28A1 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 P28A1
What does P28A1 mean?
P28A1 means "Cylinder 5 Ion Current Sense Circuit Range/Performance". P28A1 flags a cylinder 5 ion signal that is electrically intact but implausible in size or shape compared with load, speed and the other cylinders. It is a generic code, so it means the same on every make.
Is it safe to drive with P28A1?
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 P28A1 fail an emissions test?
Yes, it can. P28A1 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 P28A1 to fix?
Difficulty: easy. A typical repair takes 0.3 to 2 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P28A1 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.