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.
Medium severity. Can I keep driving?If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.
Cylinder 9 is numbered according to the manufacturer: check your engine's manual before working on it.
Symptoms
- Check engine light, often coming and going
- Fault stored but hard to reproduce in the workshop
Most 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
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P234C means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 3: Ignition and misfires
- 4C: Specific fault in that system
Frequently asked questions
Can I keep driving with code P234C?
If the car is running normally, drive in moderation to a repair shop. If it loses power or another warning appears, do not continue.
What causes code P234C?
The most common causes are: intermittent contact at the glow plug connector; harness rubbing on the cylinder head, breaking contact under vibration; sensor electronics unstable at high temperature.
Related codes
- P2348Cylinder 9 Pressure Sensor Circuit
- P234ACylinder 9 Pressure Sensor Circuit Low
- P234BCylinder 9 Pressure Sensor Circuit High
- P2349Cylinder 9 Pressure Sensor Circuit Range/Performance
- P0399Cylinder 1 Pressure Sensor Circuit Intermittent/Erratic
- P2356Cylinder 10 Pressure Sensor Circuit Intermittent/Erratic
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.