P0401: Exhaust Gas Recirculation (EGR) Flow Insufficient Detected
Updated November 12, 2026
The EGR system sends a little exhaust back into the engine to lower combustion temperatures. P0401 means the engine computer isn't seeing enough of that flow, often because of carbon buildup.
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🔧 Want to check this yourself? An OBD2 scanner reads the code; models with live data also show the sensor readings used in the checks below. See code readers and scanners from $24.99 ↓ · Which scanner do I need?
Common causes
- Carbon-clogged EGR passages or valve
- A faulty EGR valve
- A failing EGR pressure sensor (for example, the DPFE sensor on many Fords)
- Vacuum supply problems on older systems
Code meanings can vary slightly by manufacturer. Use these as a starting point and confirm with your vehicle's service information or a mechanic before replacing parts.
Symptoms
- Check engine light
- Engine pinging or knocking under load
- Failed emissions test
Is it safe to drive?
Usually drivable, but pinging under load should be fixed to protect the engine.
How to check it with a scanner
- Plug an OBD2 scanner into the port under the dashboard and read all stored codes and the freeze-frame data (the conditions when the code set).
- Command the EGR valve with a scanner that supports active tests, if yours does, and watch for an RPM change.
- Inspect and clean carbon from the EGR valve and passages.
- Check the EGR pressure sensor and its hoses.
- After a repair, clear the code and drive normally for a few days to confirm it doesn't come back.
A basic code reader shows the code; scanners with live data let you watch sensor readings, which is what you need for most of the checks above. See OBD2 scanners under $100.
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Questions
What does P0401 mean?
P0401 is the generic OBD2 code for "Exhaust Gas Recirculation (EGR) Flow Insufficient Detected." The EGR system sends a little exhaust back into the engine to lower combustion temperatures. P0401 means the engine computer isn't seeing enough of that flow, often because of carbon buildup.
Is it safe to drive with P0401?
Usually drivable, but pinging under load should be fixed to protect the engine.
How do I check P0401 myself?
Read the code and freeze-frame data with an OBD2 scanner, then: Command the EGR valve with a scanner that supports active tests, if yours does, and watch for an RPM change. Inspect and clean carbon from the EGR valve and passages. Check the EGR pressure sensor and its hoses.
Related codes
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