Powertrain, generic (extended)

P28BD Cylinder 12 Ion Current Sense Circuit Range/Performance

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modeNo

P28BD reports that cylinder 12 delivers an ion signal of acceptable voltage whose content is not credible – its strength, timing or shape after the spark does not match the operating point or the rest of the engine. Saab's later detection module illustrates why content matters: it splits each raw ion trace with filters, a band-pass for the ringing that indicates knock and a low-pass for the slower part that describes combustion quality. A distorted trace corrupts both outputs. Likely sources are an eroded or oily plug, mixture errors on cylinder 12, carbon in the chamber or reduced compression. CARB counts such plausibility checks as part of input monitoring. No manufacturer documents P28BD. Electrical faults are P28BC, P28BE and P28BF; a real misfire adds P0312.

Likely causes

Eroded or oil-wetted spark plug on cylinder 12

Mixture error on cylinder 12 (injector, intake leak at that port)

Carbon build-up in the cylinder 12 combustion chamber

Reduced compression on cylinder 12

Symptoms

MIL on after the fault repeats on a later trip
Occasional roughness, sometimes with P0312
Knock control on cylinder 12 reacts erratically

Repair estimate

DifficultyEasyDIY possible

Parts and labour€30–650

Time0.3–3 h

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

Parts involved

spark plug cyl 12ignition coil cyl 12ion sense module

Related codes

P28BC P28BE P28BF P28B9 P0312

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