P28A8 Cylinder 7 Ion Current Sense Circuit

P28A8 covers a basic circuit fault in the ion current measurement of cylinder 7, so the module obtains no evaluable signal from that plug. A cylinder 7 only exists on engines with eight or more cylinders, and Saab, the best-known user of ion sensing, applied it to four-cylinder and V6 engines; no maker publicly describes an eight-cylinder version. No manufacturer publicly documents P28A8. Its physical position depends on the maker's numbering, which differs between brands on V8 engines. The principle stays the same: a bias voltage across the plug gap lets the burning gas conduct a small current, and a lost signal blinds misfire and knock assessment for that cylinder. Diagnosis starts at coil 7, its connector and the spark plug.

Likely causes

Symptoms

• MIL with ion fault stored for cylinder 7
• Misfire and knock monitoring of cylinder 7 unavailable
• Some power loss from protective timing

Repair estimate

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

Parts involved

Related codes

References: law.cornell.edu, en.wikipedia.org, saabwisonline.com.