Mazda

The EGR system is malfunctioning-

P0401 indicates the Self Test has detected insufficient EGR flow

POSSIBLE CAUSE :

Vacuum supply.
EGR valve stuck closed.
EGR valve leaks vacuum.
EGR flow path restricted.
EGRVR circuit shorted to PWR.
VREF open to DPFEGR sensor.
DPFEGR sensor downstream hose off or plugged.
EGRVR circuit open to PCM.
VPWR open to EGRVR solenoid.
DPFEGR sensor hoses both off.
DPFEGR sensor hoses reversed.
Damaged EGR orifice tube.
Damaged EGRVR solenoid.
Damaged PCM.

HINT:

EGR System Description
Receives signals from the engine coolant temperature (ECT) sensor, intake air temperature (IAT) sensor, throttle position (TP) sensor, mass air flow (MAF) sensor, and crankshaft position (CKP) sensor to provide information on engine operating conditions to the PCM.
The engine must be warm, stable and running at a moderate load and rpm before the EGR system is activated. The PCM deactivates EGR during idle, extended wide open throttle or whenever a failure is detected in an EGR component or EGR required component.

EGR Vacuum Regulator (EGRVR) Solenoid Description
Receives a variable duty cycle signal (0 to 100%) from the PCM. The higher the duty cycle, the more vacuum the solenoid diverts to the EGR valve.

EGR Valve Description
An increase in vacuum acting on the EGR valve diaphragm overcomes the valve spring and begins to lift the EGR valve pintle off its seat, causing exhaust gas to flow into the intake manifold.

EGR Orifice Tube Description
One side of the tube is exposed to exhaust backpressure and the other side to the intake manifold, causing a pressure drop across the orifice whenever there is EGR flow. When the EGR valve closes, there is no longer any flow across the metering orifice and pressure on both sides of the orifice is the same.

Differential Pressure Feedback EGR (DPFEGR) Sensor Description
Measures the actual pressure drop across the metering orifice and relays a proportional voltage signal (0 to 5 V) to the PCM.