P28C2 code meaning
Ion Sense Module Supply Voltage High
Powertrain system Generic code P28xx codes Check engine light
What P28C2 means
P28C2 is stored when the voltage feeding the ion sense module exceeds its upper limit. Since the module derives its measuring and, in cassette designs, its ignition voltages from this feed, an excessive input can raise the bias across the plugs, push signals toward the top of their range and stress the converter inside. The cause is rarely the module: a failing alternator regulator, a jump start from a 24 V source or a poor ground at the module that makes the measured difference look larger are more typical. Saab's published ignition diagnostics have no overvoltage check at all, only the loss-of-supply codes, so there is no maker analogue for this case; the code follows the CARB demand to check inputs for out-of-range values. No manufacturer documents P28C2. P28C0 and P28C1 are the open and low faults.
P28C2 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 P28C2
- Stored overvoltage code, often together with P0563
- Flickering or unusually bright lamps, battery gassing
- Erratic misfire or knock reports on several cylinders
Common causes of P28C2
- Most likelyAlternator voltage regulator letting the system voltage rise too high
- PossibleJump start or charger with excessive voltage (e.g. 24 V)
- PossiblePoor module ground distorting the voltage reading
- Less likelyFaulty voltage measurement inside the module
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Ion sense moduleWiring harness
What each part of P28C2 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. |
| C2 | The exact fault in that area. |
What to do next
- Note P28C2 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 P28C2
What does P28C2 mean?
P28C2 means "Ion Sense Module Supply Voltage High". P28C2 is stored when the voltage feeding the ion sense module exceeds its upper limit. It is a generic code, so it means the same on every make.
Is it safe to drive with P28C2?
Drive gently and only to a workshop. With P28C2 the car can go into limp mode (reduced power) to protect itself.
Will P28C2 fail an emissions test?
Yes, it can. P28C2 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 P28C2 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 P28C2 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.