DTC P0401: EGR System - Insufficient Flow: Description

WARNING: This page does not describe the selected car, but rather 13 other vehicles, including the 2002 GMC Yukon XL, 2002 GMC Sierra, 2002 GMC Savana Special, 2002 GMC Savana Camper Special, and 2002 GMC Savana. However, it is still accessible from the selected car via links, so may be relevant.

The Powertrain Control Module (PCM) tests the Exhaust Gas Recirculation (EGR) system during deceleration by momentarily commanding the EGR valve open. The pressure in the intake manifold increases when the EGR valve is open. The PCM monitors the Manifold Absolute Pressure (MAP) sensor signal during the EGR system diagnostic in order to determine the amount of EGR flow. The PCM calculates an error based on the difference between the actual MAP increase and a predetermined value. The PCM records the error and adjusts an internal fail counter towards the fail threshold if the error is too great. The PCM sets this Diagnostic Trouble Code (DTC) when the fail counter exceeds the fail threshold. The number of test samples required to exceed the fail threshold varies according to the magnitude of the detected flow errors. The PCM allows only one EGR flow test sample to be taken during an ignition cycle. In order to aid in verifying a repair, the PCM allows more than one sample per ignition cycle until 13 samples have been obtained after a scan tool Clear Info or a battery disconnect.