P28BF Cylinder 12 Ion Current Sense Circuit High

P28BF signals that the cylinder 12 ion input sits above its permitted window. A sense wire touching a powered conductor, moisture bridging pins in the coil 12 connector or spark energy escaping inside the coil can all push it there. The consequence is easiest to see on Saab's T7 V6, where knock was read from the ion flow: a signal above the knock level made the engine computer retard that cylinder step by step, and if average retard across all cylinders grew too large it enriched the mixture and finally limited air mass. A false high reading can therefore cost power and fuel. Engine computers are expected to reject such a value and fall back to safe timing. No manufacturer documents P28BF publicly. P28BE is the low-side fault, P28C2 an overvoltage of the module supply.

Likely causes

Symptoms

• Stored high ion signal code for cylinder 12, MIL depending on the maker
• Noticeably less power from heavy timing retard
• Higher fuel consumption from protective enrichment

Repair estimate

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

Parts involved

Related codes

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