Powertrain, generic (extended)

P28AB Cylinder 7 Ion Current Sense Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P28AB is set when the cylinder 7 ion signal sits above the highest value a healthy measuring circuit can deliver. Typical triggers are a signal lead touching a supply wire, moisture bridging the coil connector, or a coil whose ignition pulse bleeds into its measuring output. A constant high reading is treacherous for diagnosis because it can look like permanent combustion and so mask real misfire. Saab's T7 V6 shows comparable logic only per bank: its codes P1313 and P1324 flag a combustion line stuck above 5 V for 200 firings. No manufacturer publishes P28AB, and a V8 or larger engine using ion sensing is not publicly described. The low-side counterpart is P28AA; P28A9 handles readings that are in range but implausible.

Likely causes

Ion signal lead of cylinder 7 touching a supply wire
Moisture bridging contacts in the coil 7 connector
Coil 7 leaking its ignition pulse into the measuring output

Symptoms

  • MIL lit with a high ion reading stored for cylinder 7
  • Genuine misfire on cylinder 7 may go unreported
  • Timing pulled back on cylinder 7, slightly less power

Repair estimate

DifficultyMediumDIY possible
Parts and labour€40–650
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

P28A8P28A9P28AAP28AFP0307

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