P28AD Cylinder 8 Ion Current Sense Circuit Range/Performance

P28AD means the cylinder 8 ion signal is electrically intact but does not fit what the computer expects for the operating point, or it deviates clearly from the other cylinders. CARB treats such rationality checks as part of the monitoring of input components next to tests for wiring faults and for values outside the valid band. The current through the gap rises with pressure and temperature of the burning charge, so weak combustion from a worn plug, a partly blocked injector or poor compression in cylinder 8 lowers it, while oil deposits can distort it. On Saab's T8 the engine computer evaluates each cylinder only inside a crank-angle window after the spark. No manufacturer publicly documents P28AD. Circuit faults are P28AC, P28AE and P28AF; a confirmed misfire adds P0308.

Likely causes

Symptoms

• MIL, often together with P0308
• Rough running under load
• Knock control intervenes more often on cylinder 8

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.