Powertrain, generic (extended)

P28B9 Cylinder 11 Ion Current Sense Circuit Range/Performance

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P28B9 stands for an ion signal from cylinder 11 whose voltage stays inside the valid band but whose behaviour makes no sense: too weak or too strong for the current speed and load, or out of line with the other eleven cylinders. Such rationality tests belong to the input-component monitoring CARB demands. Since the reading follows the heat and pressure of the burning charge, the trigger is frequently in the cylinder itself, such as a worn or oil-fouled plug, a weak injector or a leaking valve. Saab's T7 V6 shows how strict this gets: it tallied missing combustions per cylinder and compared them with an allowance taken from a speed and load table, enabled only above 72 kPa ambient pressure and -7 °C. No manufacturer documents P28B9. P28B8, P28BA and P28BB are the electrical faults.

Likely causes

Worn or oil-fouled spark plug on cylinder 11
Cylinder 11 injector delivering too little or spraying poorly
Low compression on cylinder 11 (valve or ring leakage)
Water or oil standing in the plug well of cylinder 11

Symptoms

  • MIL on after the fault is confirmed on a second trip
  • Slightly rough running, sometimes with misfire code P0311
  • Unsteady ignition timing on cylinder 11

Repair estimate

DifficultyEasyDIY possible
Parts and labour€30–650
Time0.3–3 h

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

Parts involved

spark plug cyl 11ignition coil cyl 11ion sense module

Related codes

P28B8P28BAP28BBP28BDP28B5P0311

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