Powertrain, generic (extended)

P28A9 Cylinder 7 Ion Current Sense Circuit Range/Performance

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P28A9 means cylinder 7 returns an ion signal that is electrically sound but not believable for the current speed and load, or that stands out against the other cylinders of an eight-or-more-cylinder engine. Because the current through the plug gap grows with combustion pressure and temperature, a lean or misfiring cylinder 7 produces this picture as readily as a fouled plug or moisture in the well. The signal serves both misfire detection and knock control, as Saab's ion-sensing T7 V6 illustrates by analysing each bank's ion flow for knock. No manufacturer documents P28A9 publicly, and no production V8 with this measurement is publicly described. Electrical faults on cylinder 7 are reported as P28A8, P28AA and P28AB.

Likely causes

Fouled or worn spark plug in cylinder 7
Lean mixture or misfire in cylinder 7
Moisture in the plug well of cylinder 7

Symptoms

  • MIL, possibly together with P0307
  • Uneven idle
  • Knock control retards timing more often

Repair estimate

DifficultyEasyDIY possible
Parts and labour€30–500
Time0.3–2.5 h

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

Parts involved

spark plug cyl 7ignition coil cyl 7ion sense module

Related codes

P28A8P28AAP28ABP28A5P0307

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