Powertrain, generic (extended)

P28A2 Cylinder 5 Ion Current Sense Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P28A2 is stored when the cylinder 5 ion reading stays under its lower threshold. The measurement relies on a constant bias across the plug gap between sparks – about 80 V on Saab's T7 discharge modules – and anything that drains it to ground pulls the reading down: a chafed signal conductor, a cracked plug insulator, or a coil whose bias supply has failed. On Saab's four-cylinder T7, a combustion line shorted to ground produced no dedicated fault code and surfaced only as misfire codes, which illustrates the gap such a low-side code closes. On Saab's V6 the bank 1 module serves cylinders 1, 3 and 5 together. No manufacturer publicly documents P28A2. P28A3 is the high-side fault.

Likely causes

Signal conductor of cylinder 5 chafed against ground
Cracked insulator on the cylinder 5 spark plug
Bias supply of coil 5 failed

Symptoms

  • MIL with low ion signal on cylinder 5
  • Misfire codes for cylinder 5 may appear
  • Safety ignition map reduces power

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–550
Time0.3–2.5 h

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

Parts involved

spark plug cyl 5ignition coil cyl 5ion sense module

Related codes

P28A0P28A1P28A3P28A6P0305

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