What it means
Cylinder-to-cylinder pressure variation on cylinder 5 is above the calibrated maximum — combustion is unstable cycle-to-cycle, typical of partial misfire or injector seat erosion.
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 lets the controller compare combustion pressure from cycle to cycle. P03C5 sets when that variation is higher than the calibrated maximum, often because cylinder 5 is firing inconsistently from a misfire, injector problem, ignition fault, or poor compression.
Common causes
Most likely first.
- Misfire or incomplete combustion in cylinder 5
- Fuel injector fault
- Ignition fault, where applicable
- Low compression
Symptoms
- Check-engine light
- Rough or uneven idle
- Engine misfire or hesitation
- Reduced engine power
How to diagnose it
- Confirm P03C5, record freeze-frame data, and check for cylinder 5 misfire or injector codes.
- Review live misfire counts and cylinder pressure variation to establish whether the instability is isolated to cylinder 5.
- Inspect the cylinder 5 pressure-sensor connector and harness for a loose connection or damaged wire that could create false variation.
- Check cylinder 5 injector operation and, where applicable, the ignition components serving that cylinder.
- If electrical and fuel checks do not explain the fault, test cylinder 5 compression and compare it with the manufacturer's specification.
Common fixes
- Secure or repair the cylinder 5 pressure-sensor connector and wiring if they are loose or damaged.
- Repair or replace a confirmed faulty cylinder 5 injector.
- Repair a confirmed ignition fault affecting cylinder 5, where applicable.
- Address a cylinder 5 compression or sealing problem identified by testing.
Can I keep driving?
Avoid hard acceleration and extended driving if the engine misfires, hesitates, or runs roughly; arrange diagnosis soon. A persistent misfire can increase emissions and may damage the exhaust system.
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.