Emissions
EGR Valve
The EGR valve recycles a little exhaust gas into the intake to lower combustion temperature and NOx.
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Emissions flow
Interactive — tap a partTap a part to see how it fits in the system.
What it does
- Feeds exhaust gas back into the intake at light load
- Lowers combustion temperature
- Reduces nitrogen oxide emissions
When it fails
- P0401–P0404 codes
- Rough idle
- Spark knock under load
- Failed emissions test
How to diagnose
- Test EGR command with a scan tool
- Inspect the valve for carbon
- Clean the EGR passages
- Test the vacuum solenoid if fitted
Maintenance
- Clean the EGR valve and passages on high-mileage cars
- Fix intake carbon to keep EGR working
Use the diagnosis order. When a part fails, confirm each hypothesis with measurements before replacing anything — codes and symptoms overlap.
Related part
Catalytic Converter
The catalytic converter turns harmful exhaust gases into less harmful ones.