Powertrain, generic (extended)

P28AF Cylinder 8 Ion Current Sense Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P28AF is logged when cylinder 8 delivers an ion signal pinned at or above the upper limit. Possible reasons include the signal conductor shorted to battery voltage or to another live circuit in the harness, internal leakage inside coil 8 that lets the high-voltage side couple into the measuring line, or a lost reference ground at the evaluation input. CARB requires input components to be checked for values beyond their valid limits, and a reading stuck high would otherwise feed false combustion and knock data to the engine computer. How close high voltages sit to the signal is visible in Saab's T7 discharge modules, which fired from a 400 V capacitor while holding 80 V on the plug for measurement. No manufacturer documents P28AF publicly. The low reading is P28AE.

Likely causes

Cylinder 8 signal conductor shorted to battery voltage
Internal leakage in coil 8 coupling into the measuring line
Lost reference ground at the evaluation input

Symptoms

  • MIL lit with a high ion reading for cylinder 8
  • Misfire on cylinder 8 may stay undetected
  • Safety timing reduces power somewhat

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 8ignition coil cyl 8ion sense module

Related codes

P28ACP28ADP28AEP28ABP0308

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