Powertrain, generic (extended)

P28AC Cylinder 8 Ion Current Sense Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P28AC flags a general electrical fault in the ion current path of cylinder 8, typically an open connection, so no usable signal reaches the evaluation electronics. On a V8 this is the highest-numbered cylinder, on V10 and V12 engines a middle one; which bank it sits on depends on the maker's numbering. A cylinder-specific code presumes one input per coil, as in the CDM of Saab's later Trionic T8, which receives a separate ion line from each of the four coils. Older T7 modules merged cylinders into shared detectors and could not name a single one. No manufacturer documents P28AC, and no eight-cylinder ion-sensing engine is publicly known. Check coil 8, its plug, connector and wiring first; range and level faults are P28AD, P28AE and P28AF.

Likely causes

Loose or corroded connector in the cylinder 8 ion line
Measuring stage of coil 8 failed
Broken conductor between coil 8 and the evaluation module
Cylinder 8 input of the evaluation module defective

Symptoms

  • MIL with a circuit fault stored for cylinder 8
  • No misfire or knock assessment for cylinder 8
  • Engine may run with conservative timing and lower power

Repair estimate

DifficultyMediumDIY possible
Parts and labour€50–700
Time0.3–3 h

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

Parts involved

spark plug cyl 8ignition coil cyl 8ion sense module

Related codes

P28ADP28AEP28AFP28A8P28B0P0308

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