Powertrain, generic (extended)

P28C2 Ion Sense Module Supply Voltage High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Alternator voltage regulator letting the system voltage rise too high
Jump start or charger with excessive voltage (e.g. 24 V)
Poor module ground distorting the voltage reading
Faulty voltage measurement inside the module

Symptoms

  • Stored overvoltage code, often together with P0563
  • Flickering or unusually bright lamps, battery gassing
  • Erratic misfire or knock reports on several cylinders

Repair estimate

DifficultyMediumDIY possible
Parts and labour€0–600
Time0.3–2.5 h

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

Parts involved

ion sense modulewiring harness

Related codes

P28C0P28C1P0563P0560

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