What it means
Cylinder-to-cylinder pressure variation on cylinder 5 is below the calibrated minimum — pressure stays unusually constant cycle-to-cycle, often a sign that the closed-loop combustion controller has lost authority or the sensor signal is filtered.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The cylinder 5 pressure sensor supplies a pressure trace that the combustion controller uses to assess how pressure changes from one firing cycle to the next. P03C4 sets when the calculated variation for cylinder 5 is below its minimum, suggesting an unusually flat signal or a problem with signal processing or combustion control.
Common causes
Most likely first.
- Combustion pressure sensor signal fault
- Pressure signal filtering or processing fault
- Combustion-control system fault
Symptoms
- Check-engine light
- Rough or uneven engine running
- Reduced engine power
How to diagnose it
- Confirm P03C4 and save freeze-frame data, noting whether rough running or reduced power was present.
- Compare cylinder 5's live pressure trace or variation data with the other cylinders under the same operating conditions.
- Inspect the cylinder 5 pressure sensor, connector, and harness for damage or poor contact that could flatten the signal.
- Check for related sensor, combustion-control, or module communication codes before replacing parts.
- If wiring is sound, follow the manufacturer's procedure to assess pressure-signal processing and combustion-control operation.
Common fixes
- Repair poor connections or damaged wiring at the cylinder 5 pressure sensor.
- Replace a confirmed faulty cylinder 5 pressure sensor.
- Correct a verified pressure-signal processing or combustion-control fault.
- Repair a module or communication fault only if diagnosis confirms it is affecting cylinder 5 pressure data.
Can I keep driving?
If the engine runs evenly and only the warning light is on, keep driving limited and arrange diagnosis. Avoid demanding loads if it runs roughly or loses power, since the controller may not be assessing cylinder 5 combustion accurately.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.