What do P0401 and P0404 together mean?

They are clues about the exhaust gas recirculation (EGR) system, not a verdict on a particular part. In the Ford 2005 model-year diesel document cited here, P0401 is defined as detected insufficient EGR flow; P0404 is defined as EGR control-circuit range/performance. [S1]

For that application, the EGR monitor estimates flow using mass-airflow readings and a speed-density calculation, then compares estimated flow with expected flow. Its P0404 description concerns commanded versus actual valve position differing beyond a stated threshold. [S1] That explains why the pair can raise two related questions: did the monitor see enough flow, and did valve position behave as expected? The cited definitions apply to that specific Ford application; they are not universal test criteria, and neither code alone identifies a failed component.

Not based on these codes alone. A code combination is a diagnostic clue, not proof that the EGR valve—or any other one part—has failed. The source describes how one specific vehicle application’s monitors evaluate flow and valve position; it does not establish the cause of your vehicle’s codes or prescribe a repair. [S1]

A useful next step is to give a qualified diagnostic professional the exact vehicle details and ask what evidence supports each finding. For example, ask whether the flow-related evidence and the commanded-versus-actual-position evidence were both checked against the service information for your exact vehicle. Don’t treat the Ford monitor description as a test procedure for a different year, make, model, or engine.

Write down the year, make, model, and engine, then record whether each code is current, pending, or stored if your scan report shows that information. Keep the complete report, including any freeze-frame details available, and note when the warning appeared and any changes you observed. Don’t guess at a code status or fill in missing readings.

For example, use a note like this, replacing every bracket with what you can verify:

“Vehicle: [year] [make] [model], [engine]. Codes: P0401 [status shown on report]; P0404 [status shown on report]. Freeze-frame: [attach or transcribe available report]. Warning first noticed: [date or approximate time]. Observed changes: [brief description, or ‘none noticed’]. Please assess both codes using service information for this exact application and explain what evidence supports the next step.”

Keep your role to collecting records and describing observations. Ask the qualified professional:

1. Are these code definitions and monitor descriptions applicable to my exact year, make, model, and engine?
2. What do the code status and freeze-frame context show, and what does that information not establish?
3. What vehicle-specific evidence supports the insufficient-flow finding and the range/performance finding?
4. What checks in the applicable service information would distinguish possible causes before a part is recommended?
5. If a repair is proposed, which finding supports it, and how will the concern be reassessed afterward?

You can preserve the scan report, note when the warning appeared, and describe changes you noticed. This guide does not provide a diagnostic test procedure, identify a failed part, or establish the correct repair for a particular vehicle; the cited source does not answer those questions for all applications. [S1]