P0400 is Mass Airflow Sensor failure.

This is the black cylinder in the hose between the air box and the turbo inlet. It has a mult-pin electrical connector on it. First check that the connector is plugged in securely.

If it is, chances are the MAF is bad. They have a finite lifespan and will fail. You can try MAF cleaner but it is a short term solution at best.

The MAF costs about $200 from the forum sponsor (get the Bosch, that is the actual manufacturer) and takes about 2 minutes to swap out.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech