Powertrain, generic (extended)

P234C Cylinder 9 Pressure Sensor Circuit Intermittent/Erratic

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P234C covers a cylinder 9 combustion pressure signal that jumps or briefly drops out instead of staying permanently low or permanently high. As an analogue only the Saab 9-5 diesel (a GM 2.0 CDTI) is published: its erratic codes for cylinders 1–4 (P0399 for cylinder 1) react much faster than the low/high codes, already once the signal has left its window for longer than 0.2 s while the engine runs, and they are Type B. Before a new glow plug is fitted, the test plan there verifies a 5 V reference within 4.8 to 5.2 V and less than 5 Ohm on the ground. Loose contacts at the plug connector, a harness rubbing on the cylinder head or electronics that fail when hot are typical. The code tends to come and go; for cylinder 9 no manufacturer documents it.

Likely causes

Intermittent contact at the glow plug connector
Harness rubbing on the cylinder head, breaking contact under vibration
Sensor electronics unstable at high temperature

Symptoms

  • Check engine light, often coming and going
  • Fault stored but hard to reproduce in the workshop

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€50–450
Time0.5–3 h

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

Parts involved

cylinder pressure sensor glow plugsensor wiringengine control module

Related codes

P2348P234AP234BP2349P0399P2356

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