An Exhaust Gas Recirculation (EGR) system lowers the Oxides of Nitrogen (NOx) emission levels that are caused by high combustion temperatures. This is accomplished by feeding small amounts of exhaust gases back into the combustion chamber. When the air/fuel mixture is diluted with the exhaust gases, the combustion temperatures are reduced.

A linear EGR valve is used on this system. The linear EGR valve is designed to accurately supply exhaust gases to the engine without the use of the intake manifold vacuum. The valve controls the exhaust flow going into the intake manifold from the exhaust manifold through an orifice with a PCM controlled pintle. The PCM controls the pintle position by using inputs from the Throttle Position (TP) sensor and the Manifold Absolute Pressure (MAP) sensor. The PCM then commands the EGR valve to operate when necessary by controlling an ignition signal through the PCM. This can be monitored on a scan tool as the Desired EGR Position.

The PCM monitors the results of the command through a feedback signal. By sending a 5 volt reference and a ground to the EGR valve, a voltage signal representing the EGR valve pintle position travels to the PCM. This feedback signal can also be monitored on a scan tool and is the actual position of the EGR pintle. The Actual EGR Position should always be near the commanded or the Desired EGR Position.

Due to the moisture that is associated with exhaust systems, the EGR valve may freeze and stick in colder weather at times. After the vehicle is brought into a warm shop for repairs, the valve warms and the problem disappears. By watching the Actual EGR and the Desired EGR Positions on a cold vehicle with a scan tool, you can easily verify the malfunction. Check the freeze frame data in order to determine if the DTC set when the vehicle was cold by viewing the Engine Coolant Temperature (ECT).

If a condition occurs where the EGR valve signal is pulled low (but not to zero), the PCM will learn the new lower value as zero. When the EGR valve signal returns to normal the PCM interprets this as a value indicating that the EGR valve did not close and may set this DTC.

Number(s) below refer to the step number(s) on the Diagnostic Table.

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored on the scan tool for later reference.

2. Commanding the EGR valve open determines whether the EGR system can control the EGR valve accurately and if the fault is present.

3. If the EGR valve 5 volt reference is shorted to ground, the DVM will read no voltage and an additional DTC will be set and engine performance will be poor. When this circuit is open, only a DTC P0405 will be set.

4. Jumping the 5 volt reference circuit to the signal circuit checks the signal circuit and PCM.

   The scan tool should display the Actual EGR Position as 100% if the signal circuit and PCM are OK.

6. Although the PCM and circuitry acted correctly in the previous step, a problem may still lie within the terminals which would not show up in probe type testing. Check the terminals for being backed out, improperly formed or damaged, and for poor tension.

10. All circuits to the EGR valve are OK at this point. The fault lies internally in the EGR valve and therefore must be replaced. Be sure all gasket material is removed from the EGR mounting surface. Even a small amount of material may cause a DTC P0401 to set. Refer to Exhaust Gas Recirculation Valve Replacement for on vehicle service of the EGR valve.

13. The replacement PCM must be reprogrammed and the crankshaft position system variation procedure must be performed. Refer to the latest Isuzu Technical Communication System (ITCS) procedures for PCM reprogramming and Powertrain Control Module (PCM) (General Description) for the Crankshaft Reluctor Variation Procedure .

14. Check the terminals for being backed out, improperly formed or damaged, and for poor tension.

15. Clearing the DTCs is a very important step for this diagnostic. The clearing function allows the EGR valve to relearn a new pintle position as the old position was inaccurate due to the malfunction that caused the DTC. The DTC must be cleared with the ignition switch ON, with the engine OFF or when the engine is idling. If the PCM sees a EGR command, the new pintle position will not be learned.

16. If no faults have been found at this point and no additional DTCs were set, refer to Diagnostic Aids for additional checks and information.