EGR flow fault
EGR codes mean exhaust gas is not flowing back into the intake the way the monitor expects.
P0400 is a general flow fault and P0401 is insufficient flow. Carbon in the passages or a valve that does not move are common, and the engine may ping or idle roughly.
The DTC page covers passage and valve checks. Cleaning or replacement is a result of those tests, not the first guess from the lamp alone.
A trouble code marks a condition to test. It does not by itself prove which part to replace.