What P0401 Means
The EGR system, which recirculates a small amount of exhaust gas back into the engine to reduce emissions and combustion temperature, isn't flowing enough gas.
Common Causes
- Clogged EGR passages or valve from carbon buildup (very common on older/high-mileage engines)
- Faulty EGR valve or its solenoid
- Failed EGR position sensor
- Vacuum leak affecting EGR valve operation
Symptoms You Might Notice
- Engine may hesitate or ping/knock under load
- Rough idle in some cases
- Check engine light on
- Slightly higher emissions
What to Check First
- 1
Remove and inspect the EGR valve and passages for carbon buildup — cleaning often resolves this
- 2
Test the EGR valve's operation with a scan tool if cleaning doesn't help
- 3
Check the EGR position sensor and related wiring