P04FC

What it means

P04FC is a generic code for Crankcase Ventilation System Performance. The control unit has judged that the crankcase ventilation system is not working as expected. Depending on the engine, this may be a disconnected or leaking breather hose, a blocked or stuck ventilation component, or a crankcase pressure or flow reading outside the expected range. The exact monitoring method varies by make and engine, so check the manufacturer's data for the vehicle.

Symptoms

• Engine warning light on
• Rough or unstable idle
• Oil leaks or seeping seals and gaskets
• Whistling, hissing or sucking noise from the engine bay
• Oil smell, or oil residue around the breather hoses and intake
• Increased oil consumption or oil in the intake pipework

Possible causes

• Disconnected, cracked, split or perished crankcase breather hose or connectionlikely
• Blocked, sticking or failed PCV valve or oil separator (including a blocked or frozen breather line in cold weather)likely
• Loose, damaged or incorrect oil filler cap, or a poorly seated dipstick or sealpossible
• Faulty crankcase pressure sensor, or a wiring or connector fault in its circuit or in a breather heater circuit, where fittedpossible
• Excessive crankcase pressure from engine wear, such as worn piston rings or bores, or turbocharger seal leakagepossible
• Unmetered air entering through a leaking seal or gasket, such as a rocker cover gasket, affecting the monitorpossible
• Known software or calibration issue on certain enginesrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting engine load, temperature and ambient conditions when the fault set, and check for related codes such as fuel trim or misfire faults.
2. Visually inspect all breather hoses, pipes and quick-release connectors between the crankcase, the intake and the oil separator for splits, softening, oil soaking, loose fittings and incorrect routing.
3. Check the oil filler cap, its seal and the dipstick fit, and look for oil leaks from the rocker cover and other seals.
4. Inspect the wiring and connectors for any crankcase pressure sensor or breather heater, and check for damage, corrosion or water ingress.
5. Check the PCV valve and oil separator for blockage, sludge, sticking or a damaged diaphragm, following the manufacturer's procedure, as design varies by engine.
6. Use live data to check the crankcase pressure sensor reading, where fitted, against the manufacturer's specification at idle and with the engine warm.
7. Smoke test the intake and breather system at low pressure to find leaks, following the manufacturer's guidance for the engine.
8. If the ventilation system is sound, check for excessive blow-by with a crankcase pressure or compression and leak-down assessment, and check the turbocharger for seal leakage.

How to fix

• Replace damaged, split or perished breather hoses and refit any loose connectors securely and correctly.
• Replace a blocked or faulty PCV valve or oil separator assembly as per the manufacturer's design.
• Fit a correct, serviceable oil filler cap, dipstick or seal, and renew leaking gaskets or seals.
• Repair wiring or connector faults, or replace a faulty crankcase pressure sensor or breather heater.
• Address the underlying cause of excessive blow-by, such as worn engine internals or turbocharger faults, after confirming with tests.
• Apply a manufacturer calibration update only if a bulletin confirms an outdated calibration is a known cause for this vehicle.

Can I keep driving?

Nearby codes

• P04FBGenericCrankcase Ventilation Hose Connection Sensor Range/Performance
• P04FDGenericEGR Actuator A Control Processor Performance