P0400 moderate

Exhaust Gas Recirculation Flow Malfunction

What does P0400 mean?

The P0400 code means the vehicle's onboard diagnostic system has detected: Exhaust Gas Recirculation Flow Malfunction. This is a moderate severity code.

Common Symptoms

  • Check Engine Light on
  • Engine knock or ping under load
  • Failed emissions test
  • Rough idle in some cases

Probable Causes (Ranked by Likelihood)

  • Faulty EGR valve (stuck open or closed) High
  • Clogged EGR passages High
  • Failed EGR control solenoid Moderate
  • Vacuum line failure on vacuum-operated EGR Moderate

Step-by-Step Diagnostic Procedure

  1. Test EGR valve operation by commanding it open with a scan tool at idle and watching for RPM drop.

  2. Inspect EGR valve and passages for carbon buildup. Clean with a wire brush and carburetor cleaner.

  3. Check vacuum supply to the EGR valve if it is vacuum operated (minimum 10 in Hg required).

  4. Test EGR solenoid resistance and voltage supply.

Frequently Asked Questions

What happens if I ignore P0400?

The engine will fail emissions, and under high load you may experience knock that can cause long-term engine damage. Addressing it promptly prevents both outcomes.

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