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.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- medium
- Estimated cost
- $80-$200 for EGR valve; $30-$80 for solenoid
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What Are the Symptoms of P0400?
- Check Engine Light on
- Engine knock or ping under load
- Failed emissions test
- Rough idle in some cases
What Causes P0400? (Most Likely First)
- 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
Can I Still Drive with P0400?
Yes, but fix soon. P0400 is rated moderate severity. It is not an immediate stop-driving fault, but it will not clear itself — book the repair rather than ignoring the light.
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.
What actually makes the computer set an EGR code?
The OBD II regulation requires the system to flag EGR when flow deviates from the manufacturer's specified rate — in either direction, too much or too little — by enough to push emissions past 1.5 times the FTP standards; and, where no EGR fault could do that, when there is no detectable EGR flow at all (Title 13 CCR 1968.2, section (e)(8.2)). Note that excess flow is a malfunction too, which is why an EGR code does not automatically mean a blocked passage. This is the threshold the monitor is designed around, not a description of your specific fault — the regulation says what the system must detect, not how to find it on your vehicle.
Mechanics Guide Step-by-step diagnosis
How Do You Diagnose P0400?
-
Test EGR valve operation by commanding it open with a scan tool at idle and watching for RPM drop.
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Inspect EGR valve and passages for carbon buildup. Clean with a wire brush and carburetor cleaner.
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Check vacuum supply to the EGR valve if it is vacuum operated (minimum 10 in Hg required).
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Test EGR solenoid resistance and voltage supply.
Sources 3
This page is built from documented references. Verify against your own service info before repair work.
- California Code of Regulations, Title 13 section 1968.2 (OBD II)malfunction criteria, section (e)
- SAE J2012Diagnostic Trouble Code Definitions (defines the generic OBD-II code ranges)
- Cross-checked against the SAE J2012:2002 DTC table (2026-08-27)archived scan of the SAE standard — readable copy, not SAE’s current edition
Which Vehicles Commonly Get P0400?
- Toyota Camry and Corolla
- Honda Accord and Civic
- Mazda 626
- Ford Focus
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