Emissions

P0400 — EGR Exhaust Gas Recirculation Flow Malfunction

Severity: medium Safe to drive

What P0400 means

The EGR system routes a portion of exhaust gas back into the intake to lower combustion temperatures and reduce NOx. P0400 sets when the PCM sees that actual recirculation flow does not match what it expects with the valve commanded open. The cause can be mechanical (clogged passages, stuck valve) or a sensor, vacuum or wiring problem.

Symptoms

  • Check Engine light on
  • Rough or uneven idle
  • Worse fuel economy
  • Hesitation or stumble on acceleration
  • Pinging or knock under load
  • Slight loss of power
  • Higher NOx emissions

Possible causes

  1. Carbon-clogged EGR passages or valve
  2. Faulty EGR valve (mechanical or electrical)
  3. Cracked or disconnected vacuum lines, or a bad control solenoid
  4. Faulty DPFE, EVP or MAP sensor
  5. Open, shorted or corroded wiring or connectors in the EGR circuit
  6. Rarely, PCM software or calibration issue

How to diagnose

  1. Scan all codes and read freeze frame data to see when the fault occurred
  2. Check battery voltage and sensor supply, and note any other codes
  3. Inspect EGR wiring and connectors for corrosion, damage or ground problems
  4. Test vacuum lines and the control solenoid for leaks (vacuum-operated systems)
  5. Compare commanded and actual EGR position and sensor readings in live data
  6. Remove the valve and inspect the valve and passages for carbon
  7. Confirm the valve moves freely and seals fully closed
  8. Test the DPFE, MAP and EVP sensors against factory specs

Common fixes

  • Clean the EGR valve and passages
  • Replace a faulty EGR valve
  • Replace damaged vacuum lines or the control solenoid
  • Replace a failed DPFE, EVP or MAP sensor
  • Repair wiring and connectors
  • Apply a PCM software update if the manufacturer offers one

P0400 on different brands

Ford · manuals
On Ford, the extended code P0400:9C (used in the 2024-2025 gasoline OBD monitor strategy) means insufficient EGR flow; it names the condition the monitor detected, not a confirmed failed part.

Questions and answers

Can I drive with a P0400 code?

Usually for a short time, yes, but fix it soon. Emissions and fuel use rise, you may get hesitation or knock, and carbon keeps building up.

What most often causes P0400?

Most often it is carbon in the EGR valve and passages, or a failed valve. Vacuum leaks, flow sensors and wiring come next.

Will cleaning the EGR valve fix it?

If carbon is the cause, often yes. Check wiring, vacuum and sensors first, though, or the code may come right back.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.