What are the symptoms of P0401?

Engine ping / knock at light throttle on acceleration. Rough cold start in some configurations. MIL on

Remove EGR valve; inspect and clean passages with EGR cleaner and a wire brush. Test EGR valve operation: vacuum-type — apply 15 inHg, idle should drop; electronic — command with scan tool. Ford DPFE sensor: inspect hoses for cracks, test sensor output voltage (0.5 V key-on, rises with EGR flow). Replace EGR valve if stuck or diaphragm is perforated ($25–120 DIY)

P0401

Exhaust Gas Recirculation Flow Insufficient Detected

🟡 SERVICE SOON POWERTRAIN · EGR System

EGR monitor detected exhaust gas flow is below expected rate during commanded EGR on test.

What You'll Notice

Engine ping / knock at light throttle on acceleration
Rough cold start in some configurations
MIL on

Probable Causes

Clogged EGR passages — carbon buildup blocking flow port in intake manifold
Faulty EGR valve (stuck closed or diaphragm failure on vacuum-operated type)
DPFE sensor failure (Ford-specific — differential pressure sensor)
Clogged EGR cooler (diesel engines)

Direct Repair Steps

Remove EGR valve; inspect and clean passages with EGR cleaner and a wire brush
Test EGR valve operation: vacuum-type — apply 15 inHg, idle should drop; electronic — command with scan tool
Ford DPFE sensor: inspect hoses for cracks, test sensor output voltage (0.5 V key-on, rises with EGR flow)
Replace EGR valve if stuck or diaphragm is perforated ($25–120 DIY)

Vehicles Affected

ford toyota honda chevy nissan