Powertrain, generic (extended)

P28BE Cylinder 12 Ion Current Sense Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P28BE is set when the ion reading of cylinder 12 stays below its minimum. On Saab's Trionic T8 each coil generates its own measuring voltage of about 120 V and keeps it applied to the plug; the module then reads how much current the ionised gas lets through. When that voltage is never built up – coil 12 has lost its bias stage – or leaks away through a cracked insulator, a soaked plug well or a sense lead grounded on a bracket, the evaluator sees a near-zero signal on every cycle. The engine computer then cannot confirm combustion in cylinder 12 and may report P0312 although the cylinder fires. No manufacturer documents P28BE, and twelve-cylinder engines with ion sensing are not publicly described. P28BF is the high-side counterpart, P28BC the open circuit.

Likely causes

Water or oil in the cylinder 12 plug well draining the measuring voltage
Cracked plug insulator on cylinder 12
Sense lead of cylinder 12 grounded against a bracket or the engine
Coil 12 no longer builds up its measuring voltage

Symptoms

  • MIL on with a low ion signal stored for cylinder 12
  • Misfire code P0312 without noticeable roughness
  • Fault often appears after washing the engine or in wet weather

Repair estimate

DifficultyMediumDIY 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 12ignition coil cyl 12ion sense module

Related codes

P28BCP28BDP28BFP28BAP0312

References: law.cornell.edu, saabwisonline.com, en.wikipedia.org.