LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Honda >> 2000 >> Prelude Base, Standard >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self-Diagnostics >> Diagnostic Tests >> DTC P1491: EGR System Malfunction
April 5, 2026: LEMON Manuals is launched! Read the announcement.

DTC P1491: EGR System Malfunction

  1. Perform PCM reset procedure. See PCM RESET PROCEDURE/CLEARING DTCS in SELF-DIAGNOSTICS - INTRODUCTION article. Connect SCS Service Connector to 2-pin service check connector. See Figure . Start engine and run at 3000 RPM with no loads and transmission in Park or Neutral until radiator fan comes on. Test drive vehicle for about 10 minutes. Maintain engine speed at 1700-2500 RPM. If DTC P1491 is indicated, go to next step. If DTC P1491 is not indicated, problem is intermittent. System is okay at this time. Check for poor connections or loose wires at EGR valve connector and PCM. Also check poor connections or loose wires at C282 connector, located at right shock tower. See appropriate wiring diagram in WIRING DIAGRAMS article.
  2. Disconnect vacuum hose from EGR valve. Connect vacuum gauge to vacuum hose. Start engine and allow it to idle. If vacuum does not exist, go to step  4 . If vacuum exists, disconnect EGR solenoid valve 2-pin connector. Recheck for vacuum at hose. If vacuum does not exist, go to next step. If vacuum exists, check EGR system vacuum hose routing. See VACUUM DIAGRAMS article. If vacuum hose routing is okay, replace EGR control solenoid valve.
  3. Turn ignition off. Disconnect PCM 32-pin connector "A". See Figure . Check for continuity between ground and EGR control solenoid valve 2-pin harness connector terminal No. 1 (Orange wire). If continuity exists, repair short in Orange wire between EGR control solenoid valve and PCM 32-pin connector "A" terminal No. 7. See appropriate wiring diagram in WIRING DIAGRAMS article. If continuity does not exist, substitute a known-good PCM and recheck. See SUBSTITUTING PCM under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - INTRODUCTION article. If symptom or indication goes away, replace original PCM.
  4. Connect vacuum gauge to EGR valve. Start engine and allow it to idle. Apply 8.0 in. Hg vacuum to EGR valve. If engine stalls or runs rough, and EGR valve holds vacuum, go to next step. If engine does not stall or run rough, or EGR valve does not hold vacuum, replace EGR valve.
  5. Turn ignition off. Disconnect EGR valve lift sensor 3-pin connector. Turn ignition on. Measure voltage between EGR valve lift sensor harness connector terminal No. 2 (Green/Black wire) and No. 3 (Yellow/Blue wire). If about 5 volts exist, go to step  7 . If about 5 volts does not exist, go to next step.
  6. Using backprobe set, measure voltage between PCM 16-pin connector "D" terminals No. 10 (Yellow/Blue wire) and No. 11 (Green/Black wire). See Figure . If about 5 volts exist, repair open in Yellow/Blue wire between PCM 16-pin connector "D" and EGR valve lift sensor. See appropriate wiring diagram in WIRING DIAGRAMS article. If about 5 volts does not exist, substitute a known-good PCM and recheck. See SUBSTITUTING PCM under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - INTRODUCTION article. If symptom or indication goes away, replace original PCM.
  7. Turn ignition off. Measure resistance between EGR valve lift sensor connector terminals No. 1 and 2. See Figure . If resistance is 100,000 ohms or more, replace EGR valve. If resistance is less than 100,000 ohms, go to next step.
  8. Measure resistance between EGR valve lift sensor connector terminals No. 1 and 3. See Figure . If resistance is 100,000 ohms or more, replace EGR valve. If resistance is less than 100,000 ohms, go to next step.
  9. Reconnect EGR valve lift sensor connector. Turn ignition on. Using backprobe set, measure voltage between ECM PCM 16-pin connector "D" terminals No. 9 (White/Black wire) and No. 11 (Green/Black wire). If about 1.2 volts exists, go to step  11 . If about 1.2 volts does not exist, go to next step.
  10. Using backprobe set, check for continuity between ground and ECM 16-pin connector "D" terminal No. 9 (White/Black wire). If continuity does not exist, repair open in White/Black wire between EGR valve lift sensor and ECM 16-pin connector "D". If continuity exists, repair short in White/Black wire between EGR valve lift sensor and ECM 16-pin connector "D".
  11. Turn ignition off. Disconnect EGR control solenoid valve connector. EGR control solenoid valve connector is a 2-pin connector located at left side of engine compartment, in front of shock tower. Turn ignition on. Measure voltage between ground and EGR control solenoid valve harness connector terminal No. 2 (Black/Yellow wire). If battery voltage exists, go to next step. If battery voltage does not exist, repair open in Black/Yellow wire between EGR control solenoid valve connector and ECU/EAT ECU fuse No. 14 (15-amp). ECU/EAT ECU fuse No. 14 (15-amp) is located in under-dash fuse/relay box.
  12. Reconnect vacuum gauge to EGR valve vacuum hose. Start engine and allow it to idle. Using a fused jumper wire, connect battery voltage to EGR control solenoid valve connector terminal No. 1 (Black/Yellow wire). While observing vacuum gauge, connect battery ground to EGR control solenoid valve connector terminal No. 2 (Red wire). If there is not about 8 in. Hg vacuum within one second, go to next step. If there is about 8 in. Hg vacuum within one second, go to step  14 .
  13. Turn ignition off. Inspect vacuum hoses between EGR and EGR control solenoid valve, and between EGR control solenoid valve and intake manifold for proper routing, leaks or restrictions. Repair hoses as necessary. If hoses are okay, replace EGR control solenoid valve.
  14. Turn ignition off. Reconnect EGR control solenoid valve connector and vacuum hose to EGR valve. Start engine and allow it to idle. Using backprobe set, connect a fused jumper wire between PCM 32-pin connector "A" terminals No. 7 (Orange wire) and No. 10 (Black wire). If engine does not run rough or stall, repair open in Orange wire between PCM connector terminal No. 7 and EGR control solenoid valve. If engine runs rough or stalls, substitute a known-good PCM and recheck. See SUBSTITUTING PCM under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - INTRODUCTION article. If symptom or indication goes away, replace original PCM.
scientia non olet · About LEMON Manuals