P0401 Exhaust Gas Recirculation Flow Insufficient Detected
The system that feeds a little exhaust back into the engine to lower emissions isn't flowing as much as it should.
Common causes
- Carbon-clogged EGR valve or passages
- Faulty EGR valve or solenoid
- Vacuum leak to the valve
- Faulty EGR pressure sensor (e.g. DPFE on Fords)
Symptoms
- Pinging or knocking under load
- Failed emissions test
- Sometimes no symptoms
Can I drive with it?
Usually drivable, but persistent pinging under load can damage the engine over time. Get it checked.
Tip: Carbon build-up is common – cleaning the valve and passages often fixes it.
Which scanner reads it?
Any OBD2 scanner reads it. Tools with active tests can command the EGR valve to check that it moves.
Budget tools like the ANCEL AD310 read it; tools with live data and graphing – like the BlueDriver or OBDLink MX+ – help you find the cause. Not sure which fits you? Try the Find My Scanner quiz.
In the data
How common is it? At New York’s emissions inspections in 2020–21, P0401 was the #18 code found on cars that failed – 1.0% of all the codes found.
Of the vehicle complaints owners filed with NHTSA from 2015 to 2025, 27 quote P0401 – most often from Chevrolet, Toyota, Hyundai owners. Complaints lean toward serious problems, so treat this as a sign of how often the code turns up in real reports, not how often cars get it.
Both sources are compared on the Trends page.
General information to help you understand a code – not a diagnosis. Your car’s service information or a qualified mechanic has the final word. See how we research.