Powertrain, generic (extended)

P28BC Cylinder 12 Ion Current Sense Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P28BC covers a wiring-level failure in the ion measuring path of cylinder 12 that keeps a valid signal from arriving, typically an open wire or a connector without contact. Cylinder 12 is the last one on a V12 or W12; no such engine with ion sensing is publicly known, and no manufacturer documents P28BC. On Saab's Trionic T8 each measuring channel needs two inputs at the detection module: the ion line from the coil and the ignition trigger of the same cylinder, which tells the module when to start listening. A break in either can leave the channel silent. With that, misfire counting and knock evaluation for cylinder 12 stop. The coil's primary circuit is a separate code, P0362. P28BD covers implausible signals, P28BE and P28BF the voltage levels.

Likely causes

Open ion signal wire or connector at coil 12
Ignition trigger for cylinder 12 not reaching the detection module
Measuring stage of coil 12 dead
Cylinder 12 channel of the evaluation module faulty

Symptoms

  • MIL on with a circuit fault stored for cylinder 12
  • No combustion feedback from cylinder 12
  • Engine may run with reduced ignition advance

Repair estimate

DifficultyMediumDIY possible
Parts and labour€50–750
Time0.5–3.5 h

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

Parts involved

spark plug cyl 12ignition coil cyl 12ion sense module

Related codes

P28BDP28BEP28BFP28B8P0362P0312

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