P0400

What does P0400 mean?

A general fault in exhaust-gas-recirculation flow.

Can I drive with P0400?

A steady check-engine light with the car otherwise behaving normally usually means you can drive carefully to get it diagnosed — but a flashing light, shaking, lost power, or a strong fuel smell changes that. When in doubt, treat it as more serious. See Can I Drive It? for a quick read on your exact symptoms.

Common symptoms

• Check engine light
• Rough idle or light knock
• Sometimes hesitation
• Often no obvious change

Possible causes

Possible causes include — not “the part is definitely bad”:

• Carbon-clogged EGR passages
• A stuck EGR valve
• The control circuit or a sensor

What P0400 does not prove

It does not prove the EGR valve is faulty. Carbon-clogged passages, the control circuit, or a sensor can all set an EGR flow code.

What you can safely check

• Whether the light is steady or flashing
• Whether idle is rough

What a shop may check

An example diagnostic direction, not a guaranteed procedure:

• Scan tool and freeze-frame data
• The EGR valve and passages for carbon blockage
• The EGR control and its wiring
• Flow testing where supported

What not to do

• Don’t just replace the EGR valve — passages are often the real blockage
• Don’t keep clearing the code without cleaning or testing

Generic OBD-II guidance — manufacturer-specific codes and causes vary by make, so always confirm against your vehicle. A code identifies a system or condition; it does not, on its own, prove which part has failed.