AutoDiag

P0401

Exhaust Gas Recirculation "A" Flow Insufficient Detected

Moderate· Fix soonIntermediatePowertrainGeneric, all makes

What it means

Not enough exhaust gas is flowing through the EGR system when the ECU opens the valve. Carbon build-up in the EGR valve or its passages is by far the most common cause.

Is it safe to drive?

Usually safe to drive. Without EGR the engine may ping under load, and the car will fail an emissions test.

Symptoms

  • Check engine light on
  • Pinging or knocking when accelerating or climbing hills
  • Failed emissions test (high NOx)

Probable causes

Most likely first

  1. 1Carbon-clogged EGR passages in the intake (very common on Honda, Ford and diesels)
  2. 2Stuck-closed or failed EGR valve
  3. 3Faulty DPFE or EGR pressure sensor (common on older Fords)
  4. 4Cracked vacuum hose to the EGR valve
  5. 5Faulty EGR solenoid

How to diagnose it

Work through these in order before buying parts

  1. 1Record the code and its freeze frame data (speed, load, temperature when it set), then clear it and see if it comes back.
  2. 2Command the EGR valve open at idle with a scan tool. Idle should drop and stumble; if nothing changes, flow is blocked.
  3. 3Remove the EGR valve and inspect the valve and intake passages for carbon.
  4. 4On vehicles with a DPFE sensor, check its hoses and voltage.
  5. 5Check vacuum supply to vacuum-operated valves.
  6. 6Check for technical service bulletins (TSBs) or software updates for your make and model; some codes have known factory fixes.

Common fixes

  • Clean the EGR passages and valve
  • Replace the EGR valve
  • Replace the DPFE or EGR pressure sensor and its hoses
  • Replace cracked vacuum hoses

This information is general guidance for all makes. Exact causes, specifications and procedures vary by vehicle, so confirm with your service manual or a qualified mechanic before replacing parts.