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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 58 (ENGINE CONTROLS SELF-DIAGNOSTICS - 4.3L)DIAGNOSTIC TESTSDTC P0401: EGR SYSTEM - INSUFFICIENT FLOWDESCRIPTION
1999 GMC Forward Control 5.7 R, Automatic

DTC P0401: EGR System - Insufficient Flow: Description

1999 GMC Forward Control 5.7 R, AutomaticSECTION Description
WARNING: This page is about a different car, the 2002 GMC Savana and 2002 Chevrolet Chevy Express. 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. Following a scan tool Clear Info or a battery disconnect, the PCM allows more than one sample per ignition cycle until 13 samples have been obtained in order to aid in verifying a repair.

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