Code
P0401
Emissions / EGR
EGR flow insufficient
Not enough exhaust gas is being recirculated, often due to carbon buildup.
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Most likely causes
- Clogged EGR valve or passages
- Faulty EGR valve
- Blocked EGR solenoid
Symptoms you may notice
- Check engine light
- Spark knock under load
- Failed emission test
Checks to run
- Inspect the EGR valve for carbon
- Clean EGR passages
- Test EGR operation
Tools you may need
Socket set
EGR cleaner
Always diagnose before replacing. Codes share symptoms; confirm the cause with measurements before buying parts.