P28C1 code meaning
Ion Sense Module Supply Voltage Low
Powertrain system Generic code P28xx codes Check engine light
What P28C1 means
P28C1 means the ion sense module is powered, but its supply sits below the level it needs to work reliably. That matters because such modules turn 12 V into far higher voltages: Saab's T7 cassette converted battery voltage to 400 V DC stored in a capacitor and also kept 80 V across each plug for measuring. With a sagging feed the measuring voltage and thus the ion current shrink, and every cylinder can appear to burn weakly at the same time. Typical causes are a tired battery, voltage loss across a corroded main relay contact or connector, a high-resistance ground point, or deep dips during cranking. Saab enabled its supply check only above 6 V at the main relay. No manufacturer documents P28C1. P28C0 covers a missing supply, P28C2 one that is too high.
P28C1 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 2 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P28C1
- Hard starting, especially when cold or with a weak battery
- Misfire codes on several cylinders at once
- Stored supply-voltage code, often together with system voltage codes
Common causes of P28C1
- Most likelyWeak or deeply discharged battery
- PossibleVoltage drop across a corroded main relay contact or connector
- PossibleHigh-resistance ground point of the module
- Less likelyAlternator charging too little
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 P28C1 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. |
| C1 | The exact fault in that area. |
What to do next
- Note P28C1 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 P28C1
What does P28C1 mean?
P28C1 means "Ion Sense Module Supply Voltage Low". P28C1 means the ion sense module is powered, but its supply sits below the level it needs to work reliably. It is a generic code, so it means the same on every make.
Is it safe to drive with P28C1?
Drive gently and only to a workshop. With P28C1 the car can go into limp mode (reduced power) to protect itself.
Will P28C1 fail an emissions test?
Yes, it can. P28C1 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 P28C1 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 P28C1 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.