Powertrain, generic (extended)

P28B2 Cylinder 9 Ion Current Sense Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P28B2 is stored when the ion current measured at cylinder 9 remains under its lower plausibility limit. In an ion-sensing ignition each coil both fires the plug and supplies the measuring voltage, so a coil 9 with a weak bias stage, a cracked plug insulator or a signal wire grounded in the harness all reduce the current reaching the evaluator. Because the evaluation can hardly tell a dead circuit from a cylinder that does not burn, misfire code P0309 frequently accompanies it. On Saab's four-cylinder T7, a ground short on a combustion line triggered no special code at all, only misfire reports. No manufacturer publicly documents P28B2, and no ion-sensing V10 is known. P28B3 is the high fault, P28B1 the plausibility fault.

Likely causes

Cylinder 9 signal wire grounded in the harness
Cracked insulator on the cylinder 9 plug
Coil 9 with weak bias stage

Symptoms

  • MIL with low ion reading on cylinder 9
  • P0309 often stored at the same time
  • Reduced power under load

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–600
Time0.3–3 h

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

Parts involved

spark plug cyl 9ignition coil cyl 9ion sense module

Related codes

P28B0P28B1P28B3P28AEP28B6P0309

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