P04DB: Crankcase Ventilation System Disconnected
What to do
Ask for the breather hoses to be physically inspected, not just the code read. These hoses go brittle with age and split or snap out of sight under the manifold, which reads the same as being disconnected.
Mention any oil leaks, oil consumption, or a smell of oil, and any whistling or sucking noise from the engine bay — all of which fit this fault.
If anyone has previously fitted an oil catch can or removed part of the breather system, say so. That is what this check is designed to detect, and it is an MOT and emissions problem as well as a fault code.
Do not leave it. With the crankcase not being ventilated, pressure builds and pushes oil past the crankshaft seals — which turns a hose repair into a much larger one.
What this code means
The crankcase ventilation system has been detected as disconnected.
Blow-by is unavoidable: some combustion gas always escapes past the piston rings into the crankcase. Left there it would pressurise the engine, push oil out of every seal, and contaminate the oil with fuel and acids. So it is drawn back into the intake and burned — which is what crankcase ventilation is for, and why venting it to atmosphere has been illegal for decades.
This code exists because a disconnected breather hose is both a common failure and a common modification, and manufacturers now detect it deliberately.
A disconnected hose does three things at once:
- It vents oil vapour to atmosphere, which is the emissions offence and an MOT concern.
- It admits unmetered air to the intake if the disconnection is on the intake side, so the mixture goes lean and fuel trim codes follow.
- It stops the crankcase being evacuated, so pressure rises and pushes oil past the crankshaft seals — which is how a detached hose becomes an oil leak and, eventually, a seal replacement.
Check the hose physically rather than trusting the code. These hoses become brittle with heat and age and split along their length or snap at the elbow, often out of sight underneath the manifold, and a split reads the same as a disconnection.
A hose that popped off was pushed off by pressure, so look for a blocked breather or oil separator as the underlying cause.
Likely causes
Start here
Less common
- Blocked oil separator · moderate
- Vacuum leak after the airflow sensor · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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