P28C1Ion Sense Module Supply Voltage Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- 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
- Likely causes
- LikelyWeak or deeply discharged battery
- PossibleVoltage drop across a corroded main relay contact or connector
- PossibleHigh-resistance ground point of the module
- RareAlternator charging too little
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Easy · Often possible to do yourself.Typical cost: €0 – €400
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.