P0401 EGR System Failure

NAME OF CODE
EGR System Failure

WHEN MONITORED
With engine temperature greater than 180°F, ambient temperature above 0°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. The test is repeated up to 3 times in a run cycle (2 trips). If the test fails every time, the code is set. If any of the tests pass, the diagnostics are suspended for the run cycle.

THEORY OF OPERATION
The EGR system recirculates exhaust gases back through the intake manifold to lower combustion temperature. This reduces NOx emissions and prevents spark knock. The EGR valve is controlled by the EGR solenoid. The PCM controls the solenoid operation.

POSSIBLE CAUSES
- Failed EGR valve (high or low flow)
- Failed EGR solenoid assembly
- Failed PCM
- Leaky EGR valve seat
- Plugged or leaky EGR passages