Powertrain, generic (extended)

P28A4 Cylinder 6 Ion Current Sense Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P28A4 is the general circuit code for the ion measurement of cylinder 6: the electronics get no usable signal from that plug. Only engines with six or more cylinders are affected, and among them ion sensing is rare. The V6 of Saab's 9-5 with Trionic T7 used it; there cylinder 6 is in bank 2 with 2 and 4, is triggered from engine module pin 32, and shares its start spark with cylinder 3. The V6 of the 2007 Saab 9-3 is a counterexample: its Motronic ME9 relies on two piezo knock sensors, one per bank, so no ion signal exists there at all. No manufacturer publicly documents P28A4; on an affected engine, coil 6, its wiring and the plug are checked first.

Likely causes

Ion measuring part of the cylinder 6 coil defective
Open ion signal circuit of cylinder 6 in the harness or connector
Processing input for cylinder 6 failed

Symptoms

  • MIL with ion fault stored for cylinder 6
  • Reduced misfire and knock monitoring on cylinder 6
  • Moderate power loss from cautious timing

Repair estimate

DifficultyMediumDIY possible
Parts and labour€60–650
Time0.3–2.5 h

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

Parts involved

spark plug cyl 6ignition coil cyl 6ion sense module

Related codes

P28A5P28A6P28A7P28A0P0306

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