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DTC P1491: Insufficient EGR Valve Lift

  1. Perform PCM reset procedure. See PCM RESET PROCEDURE/CLEARING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. Start engine and run at 3000 RPM, with no load and transmission in Park or Neutral, until radiator fan comes on. Check temporary DTC with scan tool. Test drive vehicle for about 10 minutes. Keep engine speed at 1700-2500 RPM. If DTC P1491 resets, go to next step. If DTC 1491 does not reset, problem is intermittent. System is okay at this time. Check for poor connection(s) or loose wire(s) between PCM and EGR valve. Repair as necessary.
  2. Turn ignition off. Disconnect EGR valve 6-pin harness connector. Start and idle engine. Measure voltage between EGR valve 6-pin harness connector terminals No. 4 (Blue/Red wire) and No. 6 (Black wire). See WIRING DIAGRAMS article. Repair as necessary. If battery voltage is not present, go to next step. If battery voltage is present, substitute known-good PCM, then recheck. If symptom or indication goes away, replace original PCM.
  3. Turn ignition off. Turn ignition back on. Measure voltage between EGR valve 6-pin harness connector terminal No. 2 (Light Green/Yellow wire) on MDX or (Green/Yellow wire) and terminal No. 3 (Yellow/Blue wire). If about 5 volts are not present, go to next step. If about 5 volts are present, go to step  5.
  4. Measure voltage between PCM 31-pin harness connector terminals No. 28 (Yellow/Blue wire) and No. 18 (Light Green/Yellow wire) on MDX or (Green/Yellow wire) 3.2CL and 3.2TL. If about 5 volts are present, repair open in Light Green/Yellow wire on MDX or Green/Yellow wire on 3.2CL and 3.2TL between PCM 31-pin harness connector terminal No. 18 and EGR valve 6-pin harness connector terminal No. 2. If about 5 volts are not present, substitute known-good PCM, then recheck. If symptom or indication goes away, replace original PCM.
  5. Turn ignition off. Measure resistance between EGR valve 6-pin connector terminals No. 1 (White/Black wire) and No. 2 (Green/Yellow wire). See WIRING DIAGRAMS article. Repair as necessary. If less than 100 k/ohms are present, go to next step. If more than 100 k/ohms are present, replace EGR valve.
  6. Measure resistance between EGR valve 6-pin connector terminals No. 1 (White/Black wire) and No. 3 (Yellow/Blue wire). If less than 100 k/ohms are present, go to next step. If more than 100 k/ohms are present, replace EGR valve.
  7. Reconnect EGR valve 6-pin connector. Turn ignition on. Measure voltage between PCM 31-pin harness connector terminals No. 6 (White/Black wire) and No. 18 (Green/Yellow wire). If about 1.2 volts are present, go to next step. If about 1.2 volts are not present, check continuity between ground and PCM 31-pin harness connector terminal No. 6 (White/Black wire). If continuity is present, repair short in White/Black wire between PCM 31-pin harness connector terminal No. 6 and EGR valve 6-pin harness connector terminal No. 1. If continuity is not present, repair open in White/Black wire between PCM 31-pin harness connector terminal No. 6 and EGR valve 6-pin harness connector terminal No. 1.
  8. Turn ignition off. Disconnect EGR valve 6-pin harness connector. Using fused jumper wire, connect battery voltage to EGR valve 6-pin connector terminal No. 4 (Blue/Red wire). See WIRING DIAGRAMS article. Repair as necessary. Start and idle engine. Using second fused jumper wire, connect ground to EGR valve 6-pin connector terminal No. 6 (Black wire). If engine stalls or runs rough, go to next step. If engine does not stall or run rough, replace EGR valve.
  9. Turn ignition off. Disconnect PCM 25-pin harness connector. Check continuity of Blue/Red wire between PCM 25-pin harness connector terminal No. 7 and EGR valve 6-pin harness connector terminal No. 4. If continuity is present, go to next step. If continuity is not present, repair open in Blue/Red wire between PCM 25-pin harness connector terminal No. 7 and EGR valve 6-pin harness connector terminal No. 4.
  10. Check continuity between ground and PCM 25-pin harness connector terminal No. 7 (Blue/Red wire). If continuity is not present, go to next step. If continuity is present, repair short to ground in Blue/Red wire between PCM 25-pin harness connector terminal No. 7 and EGR valve 6-pin harness connector terminal No. 4.
  11. Check continuity between ground and EGR valve 6-pin harness connector terminal No. 6 (Black wire). If continuity is present, substitute known-good PCM, then recheck. If symptom or indication goes away, replace original PCM. If continuity is not present, repair open in Black wire between ground and EGR valve 6-pin harness connector terminal No. 6.
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