Powertrain, generic (extended)

P28A1 Cylinder 5 Ion Current Sense Circuit Range/Performance

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P28A1 flags a cylinder 5 ion signal that is electrically intact but implausible in size or shape compared with load, speed and the other cylinders. Ion current rises with chamber pressure and temperature, so a lean or poorly compressing cylinder 5 produces a weak trace exactly like a fouled plug does. Saab's T7 V6 shows how closely the signal is tied to misfire detection: its engine module accumulates missing combustion signals per cylinder over 1000 revolutions and compares them with a load- and speed-dependent limit before setting P0305. That discharge module also evaluates knock from the ion flow of its bank. No manufacturer documents P28A1 publicly; electrical faults on cylinder 5 are P28A0, P28A2 and P28A3.

Likely causes

Fouled or eroded spark plug in cylinder 5
Lean mixture in cylinder 5 from a clogged injector or intake leak
Low compression in cylinder 5

Symptoms

  • MIL, possibly with misfire code P0305
  • Uneven running under load
  • Higher fuel consumption

Repair estimate

DifficultyEasyDIY possible
Parts and labour€30–450
Time0.3–2 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

P28A0P28A2P28A3P28A5P0305

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