Powertrain, generic (extended)

P28C1 Ion Sense Module Supply Voltage Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Weak or deeply discharged battery
Voltage drop across a corroded main relay contact or connector
High-resistance ground point of the module
Alternator charging too little

Symptoms

  • 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

Repair estimate

DifficultyEasyDIY possible
Parts and labour€0–400
Time0.3–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

ion sense modulewiring harness

Related codes

P28C0P28C2P0562P0560

References: law.cornell.edu, saabwisonline.com, saabwisonline.com.