P0400 moderate

Exhaust Gas Recirculation Flow Malfunction

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

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Keep driving?: Yes, but fix soon
DIY difficulty: medium
Estimated cost: $80-$200 for EGR valve; $30-$80 for solenoid

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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

Test EGR valve operation by commanding it open with a scan tool at idle and watching for RPM drop.
Inspect EGR valve and passages for carbon buildup. Clean with a wire brush and carburetor cleaner.
Check vacuum supply to the EGR valve if it is vacuum operated (minimum 10 in Hg required).
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.