Test Description

The numbers below refer to the step numbers in the diagnostic procedures.

1. 1
   
   Powertrain OBD II system check prompts technician to complete some basic checks and store the freeze frame and failure records data on scan tool if applicable. This creates an electronic copy of data taken when fault occurred. The information is then stored on the scan tool for later reference.
2. 2
   
   Commanding EGR valve OPEN determines whether EGR system can control EGR valve accurately and if the fault if present.
3. 3
   
   When the EGR valve harness connector is disconnected, the scan tool should display actual EGR position as 0%. If it does not, fault lies either in the EGR signal circuit or PCM.
4. 4
   
   A test light, when connected to ground, will glow dimly when the EGR valve is commanded to 25%, and brighter as the EGR valve is commanded to 100%. If test light flashes, check EGR sensor ground for an open.
5. 5
   
   An open or poor connection condition may have caused this DTC to set. Check harness connector terminals for being backed out, improperly formed or damaged, or for poor tension.
6. 7
   
   Test light will have glowed brightly in previous step if EGR control circuit was shorted to battery voltage and actual EGR position on scan tool will display 100%. A test light that did not illuminate, indicates that circuit may be open or shorted to ground.
7. 9
   
   If EGR valve 5-volt reference is shorted to voltage, DVOM will read battery voltage and additional DTCs may be set, and engine performance will be poor.
8. 12
   
   Replacement PCM must be reprogrammed and Tooth Error Correction (TEC) procedure must be performed. See POWERTRAIN CONTROL MODULE REPLACEMENT  and TOOTH ERROR CORRECTION LEARN PROCEDURE  under POWERTRAIN CONTROL MODULE.
9. 13
   
   Although circuitry acted correctly when checked, a problem may still lie within terminals which would not show up in probe type testing. Be sure to check terminals for being backed out, improperly formed or damaged, and for poor terminal tension.
10. 17
    
    All circuits to the EGR valve are okay at this point. Fault lies internally in EGR valve. EGR valve must be replaced, or tested with a known good unit. When replacing EGR valve, ensure all gasket material is removed from EGR mounting surface. Even a small amount of material may cause a DTC P0401 to set.
11. 18
    
    Check harness connector terminals for being backed out, improperly formed or damaged, and for poor terminal tension.
12. 19
    
    Clearing DTCs is a very important step for this diagnostic. Clearing function allows EGR valve to relearn a new pintle position as old position was inaccurate due to malfunction that caused DTC to set. DTC must be cleared with ignition switch in ON position, with the engine OFF or when engine is idling. If PCM sees a EGR command, the new pintle position will not be learned.