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Reset the PCM . 2. Start the engine. Hold the engine at 3,000 rpm with no load (in Park or neutral) until the radiator fan comes on. 3. Drive the vehicle under load for about 10 minutes . Try to keep the engine speed in the 1,700 – 2,500 rpm range. 4. Check the Temporary DTC with the scan tool. Is Temporary DTC P1491 indicated? YES – Go to step 5. NO – Intermittent failure, system is OK at this time. Check for poor connections or loose wires at the EGR valve and at the PCM. 5. Turn the ignition switch OFF . 6. Disconnect the EGR valve 6P connector. 7. Start the engine and let it idle. 8. Measure voltage between EGR valve 6P connector terminals No. 4 and No. 6. Is there about 9 – 10 V ? YES – Go to step 9. NO – Substitute a known-good PCM, and recheck. If the symptom/indication goes away, replace the original PCM. 9. Turn the ignition switch OFF . 10. Turn the ignition switch ON (II). 11. Measure voltage between EGR valve 8P connector terminals No. 2 and No. 3. Is there about 5 V ? YES – Go to step 13. NO – Go to step 12. 12. Measure voltage between PCM connector terminals C18 and C28. Is there about 5 V? YES – Repair open in the wire between the EGR valve and the PCM (C18, C28). NO – Substitute a known-good PCM and recheck. If the symptom/indication goes away, replace the original PCM. 13. Turn the ignition switch OFF .