Powertrain, generic (extended)

P28AA Cylinder 7 Ion Current Sense Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P28AA reports that the ion current reading from cylinder 7 has dropped below the floor the engine computer accepts. Ion sensing places a measuring voltage on the spark plug between firings; Saab's Trionic T8 coils generate about 120 V for this themselves. If that voltage leaks away through a carbon track inside the plug boot, a split insulator or a signal lead rubbing on the block, the evaluator sees almost no current even when the mixture burns normally. Because the reading then resembles a dead cylinder, a misfire code such as P0307 may be stored alongside. Cylinder 7 exists only on V8 and larger engines, and no maker publicly documents an ion-sensing engine of that size or this code. P28AB covers the opposite fault, P28A8 the open circuit.

Likely causes

Carbon track inside the cylinder 7 plug boot
Ion signal lead of cylinder 7 rubbed through to ground
Split ceramic insulator on the cylinder 7 plug
Measuring-voltage stage in coil 7 no longer working

Symptoms

  • MIL lit with a low ion reading stored for cylinder 7
  • Misfire code P0307 may appear although the engine runs smoothly
  • Reduced power from protective ignition timing

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–600
Time0.3–2.5 h

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

Parts involved

spark plug cyl 7ignition coil cyl 7ion sense module

Related codes

P28A8P28A9P28ABP28AEP0307

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