P0400: OBD-II code P0400 flags an EGR flow fault — the PCM commanded EGR but doesn't see the expected change in engine load or flow feedback.

Symptoms

• Check engine light is on; the engine may ping or knock under light load, and NOx emissions rise noticeably.
• Rough idle if EGR is stuck open, or reduced power if EGR is stuck closed.
• The vehicle will fail an OBD-II emissions test until the EGR monitor completes clean during a drive cycle.

Common causes

• A carbon-clogged EGR valve or passages that mechanically restrict flow.
• A failed EGR position sensor or DPFE (differential pressure feedback EGR) sensor giving bad feedback.
• A stuck-closed EGR solenoid or a failed EGR valve motor on electronic designs.

What to do?

Command the EGR valve open with a scan tool and confirm the engine responds (idle should stumble slightly). Inspect the valve and passages for carbon buildup. Test the EGR position or DPFE sensor. Clean or replace the EGR valve and clear carbon from the passages.