Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 79 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.4L)DIAGNOSTIC TESTSDTC P0401: EGR SYSTEMDIAGNOSTIC AIDS
1999 GMC Forward Control 5.7 R, Automatic
Diagnostic Aids
1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Aids
WARNING: This page is about a different car, the 2002 Pontiac Grand Am, 2002 Oldsmobile Alero, 2002 Chevrolet Monte Carlo, and 2002 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.
Check for any of the following conditions:
- Vacuum restriction to the MAP sensor. A restriction to MAP sensor can cause EGR flow test to fail due to insufficient MAP changes being monitored during the test. Inspect for objects blocking the vacuum to MAP sensor.
- An engine that is running poorly may cause this DTC to set.
- Poor connection or damaged harness. Inspect wiring harness for damage.
- If the harness appears to be okay, observe the EGR position sensor display on the scan tool while moving the connectors and the wiring harnesses that are related to the EGR valve. A change in the display indicates the location of the malfunction.
In order to verify a repair, clear info with the scan tool and run the EGR flow test, keeping the following conditions in mind:
- The PCM will only run the EGR flow test during a gradual deceleration.
- The PCM will only run the EGR test during a closed throttle condition.
- The PCM will only run the EGR test at vehicle speeds greater than 35 MPH.
- Several deceleration cycles, typically 9-12, will be necessary in order to run a sufficient number of EGR flow test samples.
- The EGR Test Counter displayed on the scan tool can be useful in determining that the EGR flow test is running, and to track the number of test samples taken. The counter will increment each time a test sample is taken.
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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.