P0401

Tests 1 - 2:

Tests 3 - 9:

SYMPTOM
P0401 - EGR SYSTEM FAILURE

WHEN MONITORED
With engine coolant temperature greater than 180 °F, ambient temperature above 40 °F, engine in closed loop, O2 sensor operational, engine under a steady load with the throttle open, and O2 controller not at or near its limits.

SET CONDITION
Once all the conditions are met, the EGR is turned OFF and ON momentarily. The oxygen sensor signal is monitored for changes in its output signal during this test. If no significant change or too much change is seen, a flag is set.

POSSIBLE CAUSES
- Exhaust leak
- Intake manifold restricted
- EGR back pressure transducer defective
- EGR valve assembly defective
- EGR valve assembly diaphragm defective
- EGR valve assembly mounted incorrectly
- EGR transducer line blocked
- Vacuum supply to EGR solenoid inadequate