P0400

A model year appears here because the code is named in its record, not because every one of these vehicles sets it. Counts are mentions in the federal files, not failure rates — nobody publishes the denominator.

Owners’ own words, one per nameplate, scrubbed of anything identifying. These are unverified reports filed with NHTSA — they are what was said, not a finding that it was true.

Service bulletins and campaigns whose filed summary names P0400. Quoting the document number at a service desk is often the fastest route to a diagnosis someone has already written down.

• TSB-23-13-004
  2023-08-02 · Engine And Engine Cooling
• This Technical Service Bulletin provides instructions to reprogram the ECU with the countermeasure software when the check engine light illuminates with DTC of P0401/ P0400.
• 11-200-20R
  2020-05-06 · Electrical System — Software
• This bulletin announces the availability of reprogramming files to optimize the Engine Control Module (ECM) for hard starting in sub-freezing temperatures, DTC P0400, and surging in high altitudes.
• 11-197-20
  2020-02-11 · Electrical System
• This bulletin announces availability of reprogramming files to optimize the Engine Control Module (ECM). These new files address concerns of the check engine light coming on and setting a DTC P0400: EGR “Aâ€Â� FLOW. The DTC can set if the ECM tests the EGR system when ambient / intake air temperatures are below freezing.

A stored code names the test that failed, never the part that caused it. It is the starting point of a diagnosis, not its conclusion — the same code has several possible causes and the cheap one is not always right.