PCM tests exhaust gas recirculation valve by momentarily commanding valve on while monitoring engine RPM. PCM will illuminate Malfunction Indicator Light (MIL) and store Code P0401 if expected drop is not seen for a calibrated number of tests.

PCM will run EGR test when the following conditions are present.

Codes P0122, P0123 or P0501 are not set.
Brake has been applied.
Engine coolant temperature is greater than 185°F (85°C).
Throttle angle is less than one percent.
Vehicle speed is greater than 25 MPH.
Engine speed is 900-1100 RPM.

Code P0401 will set when the following conditions are present.

EGR flow test will be failed and Code P0401 set if PCM does not detect a decrease in engine speed of at least 50 RPM for 6 out of 8 tests. Once 3 tests are run during an ignition cycle, PCM will not run any further flow tests until next ignition cycle. Thus, 3 or more ignition cycles will be necessary to set Code P0401. Code P0401 will illuminate Malfunction Indicator Light (MIL).

NOTE: Test numbers refer to numbers on diagnostic chart.

Ensures EGR valve is allowing EGR flow and PCM is capable of controlling EGR valve.
Ensures control circuit is not shorted to ground.
Ensures control circuit is not open and that PCM quad-driver for circuit is properly functioning.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Code P0401, (EGR) Flow Test Failure: Notes

Use the Manual With the Right Hardware

When to See a Mechanic

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