P0401 Exhaust Gas Recirculation Flow Insufficient Detected
The PCM has commanded EGR flow but the measured flow (via the EGR position sensor, DPFE sensor, or MAP delta) is below the expected threshold.
Definition
P0401 indicates the EGR system is flowing less exhaust gas than commanded. This is almost always carbon buildup in the EGR passages, but can also be a stuck-closed EGR valve or a failed DPFE/EGR position sensor.
Common causes
- Carbon buildup in EGR passages or intake manifold
- EGR valve stuck closed
- Failed DPFE (Differential Pressure Feedback EGR) sensor
- Clogged EGR tube
- Vacuum supply issues (on vacuum-actuated EGR systems)
Common misdiagnoses
- Replacing the EGR valve without cleaning the passages — the new valve will clog quickly.
Troubleshooting steps
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1. Inspect and clean EGR passages
Remove the EGR valve and intake plenum if necessary; clean carbon deposits from the EGR tube and intake ports.
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2. Test EGR valve operation
Apply vacuum or command via scan tool and verify the valve opens/closes.
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3. Verify DPFE sensor voltage
Compare commanded vs. measured EGR flow on a scan tool.