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Home › Mazda › 2003 › B4000 Base, Automatic › Repair and Diagnosis › Engine Performance › System › Engine Controls - Self-Diagnostics › Diagnostic Tests › DTC P0401: EGR System Malfunction › Diagnostic Procedures

Diagnostic Procedures

  1. If QUICK TEST was not performed, go to QUICK TEST  under SELF-DIAGNOSTIC SYSTEM. If QUICK TEST was performed:
    • For Continuous Memory DTC P1406, go to DTC P1405 OR DTC P1406  test procedure.
    • If KOER DTC P1408 does not appear with P0401, go to step  12 .
    • For all others, go to next step.
    2. NOTE: If this DTC was set by an intermittent condition, maximum benefit will be obtained by having another technician wiggle harness and connectors when making measurements.
  2. Run KOER Self-Test While Monitoring EGR Vacuum -  Disconnect vacuum hose at EGR valve. Connect a vacuum gauge to vacuum hose. With EGR vacuum hose disconnected, ignore any DTCs during this KOER Self-Test. With KOER Self-Test, monitor vacuum gauge. About 30 seconds into self-test, EGR flow will be requested for a few seconds. Vacuum should increase to more than 1.6 in. Hg (5.4 kPa) to open EGR valve. Observe vacuum gauge. If vacuum increased to 3.0 in. Hg (10 kPa) or more at anytime during KOER Self-Test, indicated vacuum is sufficient to open EGR valve. Fault is unlikely to be in EGR vacuum control system. Go to next step. If vacuum did not increase to 3.0 in. Hg (10 kPa) or more at anytime during KOER Self-Test, indicated vacuum is insufficient to open EGR valve. Go to step  6 .
  3. Inspect DPFEGR Sensor Pressure Hoses -  Inspect both pressure hoses for reversed connection at DPFEGR sensor and at orifice tube assembly. Inspect both hoses for proper routing. Verify hoses are not pinched or have dips where water could collect and freeze. Inspect both hoses for leaks or obstructions. Inspect DPFEGR sensor and orifice tube assembly for blockage or damage at pick up tubes. If any faults are found, repair hoses as necessary. Go to step  17 . If no faults are found, go to next step.
  4. Check DPFEGR Sensor Output By Applying Vacuum With Hand Pump -  Disconnect pressure hoses at DPFEGR sensor. Connect a hand vacuum pump to downstream connection at sensor (intake manifold side of sensor, or smaller diameter pickup tube). With KOEO, access DPFEGR PID. Apply 8-9 in. Hg (27-30 kPa) vacuum to DPFE sensor and hold for a few seconds. Quickly release vacuum. Observe DPFEGR PID voltage:
    • DPFEGR PID voltage must be 0.2-1.3 volts with ignition on and no vacuum applied.
    • DPFEGR PID voltage must increase to more than 4 volts with vacuum applied.
    • DPFEGR PID must drop to less than 1.5 volts in less than 3 seconds when vacuum is released.

    Turn ignition off. If DPFEGR PID voltage indicates a fault in DPFE sensor, replace faulty DPFEGR sensor. Go to step  17 . If DPFEGR PID voltage does not indicate a fault in DPFE sensor, reconnect pressure hoses. Go to next step.

  5. Check EGR Valve Function By Applying Vacuum With Hand Pump -  Disconnect vacuum hose at EGR valve and plug hose. Connect a hand vacuum pump to EGR valve. Start engine and idle. Access DPFEGR and RPM PIDs. Slowly apply 8-10 in. Hg (27-34 kPa) of vacuum to EGR valve and hold for 10 seconds. If engine attempts to stall, increase RPM with throttle to maintain a 1000 RPM minimum. Observe for the following:
    • EGR valve starts opening at about 1.6 in. Hg (5.4 kPa) vacuum indicated by increasing DPFEGR PID voltage.
    • DPFEGR PID voltage increasing until EGR valve is fully open. DPFEGR PID should read 2.5 volts minimum with full vacuum applied.
    • DPFEGR PID voltage steady when vacuum is held. If voltage drops within a few seconds, EGR valve or vacuum source may be leaking.
    If DPFEGR PID voltage indicates EGR valve is operating as described, restore vehicle to original condition. Go to step  11 . If DPFEGR PID voltage does not indicate EGR valve is operating as described, remove and inspect EGR valve for signs of contamination, unusual wear, carbon deposits, sticking, leaking diaphragm or other damage. If EGR valve is okay, look for an obstructed EGR port in intake manifold or plugged orifice tube assembly. Repair as necessary. After repair, go to step  17 .
  6. Check Vacuum Source & Vacuum Hoses To & From EGR Vacuum Regulator (EGRVR) Solenoid -  Inspect vacuum lines between vacuum source and EGRVR solenoid, and between EGRVR solenoid and EGR valve for leaks, blockage, proper routing and any damage. Disconnect vacuum hoses at EGRVR solenoid. Connect EGRVR solenoid vacuum source hose to a vacuum gauge. With engine idling at operating temperature, observe vacuum gauge. If vacuum reading is a minimum of 15 in. Hg (51 kPa) at idle and vacuum lines are okay, restore vehicle. Go to next step. If vacuum reading is not a minimum of 15 in. Hg (51 kPa) at idle and/or vacuum lines are not okay, repair as necessary. After repair, go to step  17 .
  7. Check VPWR To EGR Vacuum Regulator (EGRVR) Solenoid -  Disconnect EGRVR solenoid. With KOEO, measure voltage of VPWR circuit (Light Blue/Orange wire) between EGRVR solenoid harness connector and chassis ground. See Fig 1 . If voltage reading is more than 10.5 volts, turn ignition off. Go to next step. If voltage reading is 10.5 volts or less, repair open in EGRVR solenoid VPWR circuit. See appropriate WIRING DIAGRAM under ENGINE PERFORMANCE in SYSTEM WIRING DIAGRAMS. After repair, go to step  17 .
    Fig 1: Identifying EGRVR Solenoid Harness Connector
    G00155544Courtesy of MAZDA MOTORS CORP.
  8. Measure Resistance Across EGR Vacuum Regulator (EGRVR) Solenoid -  Measure resistance across EGRVR solenoid. If resistance reading is 26-40 ohms, go to next step. If resistance reading is not 26-40 ohms, replace faulty EGRVR solenoid. After repair, go to step  17 .
  9. Check EGR Vacuum Regulator (EGRVR) Circuit For Short To PWR -  Turn ignition off. Disconnect PCM 104-pin harness connector. Connect Breakout Box (49-UN01-130) to PCM harness connector only. See Figure . With KOEO, measure voltage between breakout box terminal No. 47 (EGRVR circuit) and chassis ground. Turn ignition off. If voltage reading is more than one volt, repair short circuit. See appropriate WIRING DIAGRAM under ENGINE PERFORMANCE in SYSTEM WIRING DIAGRAMS. After repair, go to step  17 . If voltage reading is one volt or less, go to next step.
  10. Check EGR Vacuum Regulator (EGRVR) Circuit For Open In Harness -  Measure resistance of EGRVR circuit (Brown/Pink wire) between breakout box terminal No. 47 and EGRVR solenoid harness connector. If resistance reading is less than 5 ohms, go to next step. If resistance reading is 5 ohms or more, repair open circuit in Brown/Pink wire between PCM 104-pin harness connector terminal No. 47 and EGRVR harness connector. After repair, go to step  17 .
  11. Check EGR Vacuum Regulator (EGRVR) Solenoid Vacuum Output Capacity By Grounding EGRVR Circuit -  Remove breakout box. Reconnect PCM and EGRVR solenoid. Disconnect vacuum hose at EGR valve. Connect a vacuum gauge to EGR valve vacuum hose. With KOER and engine idling, connect fused jumper wire to breakout box terminal No. 47 and chassis ground. Observe vacuum gauge. If vacuum reading is 4 in. Hg (13.5 kPa) or more, replace faulty PCM. After repair, go to step  17 . If vacuum reading is less than 4 in. Hg (13.5 kPa), replace faulty EGRVR solenoid. After repair, go to step  17 .
  12. Inspect EGR System For Intermittent Failure -  Visually inspect EGR system for signs of an intermittent failure. If a fault is found, repair fault as necessary. After repair, go to step  17 . If no fault is found, go to next step.
  13. Inspect DPFEGR Sensor Pressure Hoses -  Inspect both pressure hoses for reversed connection at DPFEGR sensor and at orifice tube assembly. Inspect both hoses for proper routing. Verify hoses are not pinched or have dips where water could collect and freeze. Inspect both hoses for leaks or obstructions. Inspect DPFEGR sensor and orifice tube assembly for blockage or damage at pick up tubes. If any faults are found, repair hoses as necessary. After repair, go to step  17 . If no faults are found, go to next step.
  14. Check DPFEGR Sensor Output By Applying Vacuum With Hand Pump -  Disconnect pressure hoses at DPFEGR sensor. Connect a hand vacuum pump to the downstream connection (intake manifold side, smaller diameter pick up tube) at sensor. With KOEO, access DPFEGR PID. Observe DPFEGR PID. Voltage reading should be 0.2-1.3 volts. Apply 8-9 in. Hg (27-30 kPa) vacuum to DPFEGR sensor. Hold vacuum for a few seconds. Observe DPFEGR PID. Voltage reading should be more than 4 volts. While observing DPFEGR PID, quickly release vacuum. Voltage reading should be less than 1.5 volts in less than 3 seconds. If voltage readings are within specification, go to next step. If voltage readings are not within specification, replace faulty DPFE sensor. After repair, go to step  17 .
  15. Check EGR Valve Function By Applying Vacuum With Hand Pump -  Disconnect vacuum hose at EGR valve and plug hose. Connect a hand vacuum pump to EGR valve. Start engine and idle. Access DPFEGR and RPM PIDs. Slowly apply 5-10 in. Hg of vacuum to EGR valve and hold for 10 seconds. If engine attempts to stall, increase RPM to maintain a minimum of 800 RPM. As EGR valve begins to open, DPFEGR PID will increase. Voltage will continue to increase until EGR valve is fully open. With full vacuum applied, voltage reading should be 2.5 volts minimum. DPFEGR PID voltage should be stable when vacuum is held. If voltage reading drops within a few seconds, EGR valve or vacuum source is leaking. Turn ignition off. If voltage readings are as specified, go to next step. If voltage readings are not as specified, remove and inspect EGR valve for signs of contamination, unusual wear, carbon deposits, binding, diaphragm leaking and other damage. If EGR valve is okay, inspect for obstructed EGR port in intake manifold. Repair as necessary. After repair, go to step  17 .
  16. Inspect EGR Vacuum Signal Supply For Intermittent Failure -  Disconnect PCM 104-pin harness connector. Connect Breakout Box (49-UN01-130) to PCM harness connector and PCM. See Figure . Disconnect vacuum hose at EGR valve. Connect a vacuum gauge to disconnected hose. With KOER, jumper breakout box terminal No. 47 to ground to activate EGR vacuum regulator solenoid to full ON. At idle, vacuum gauge should be more than 4 in. Hg (13.5 kPa). Observe vacuum gauge for indication of a fault while lightly tapping EGR vacuum regulator solenoid, wiggling EGR vacuum regulator solenoid, wiggling EGR vacuum regulator solenoid connector, vacuum lines, and vehicle harness between solenoid and PCM. A sudden drop in vacuum indicates a fault is detected. If a fault is detected, isolate fault and repair as necessary. After repair, go to next step. If no fault is indicated, fault is not present at this time. In cold climates it is possible for EGR valve to freeze closed and thaw when engine warms up causing an intermittent failure. For further diagnostics, see INTERMITTENT  testing under SYSTEM TESTS.
  17. Verify Trouble Shooting Of DTCs Completed -  Reconnect all electrical connectors. Clear DTCs from PCM memory using scan tool. Perform QUICK TEST  under SELF-DIAGNOSTIC SYSTEM. If any DTC is present, replace PCM. Repeat QUICK TEST  under SELF-DIAGNOSTIC SYSTEM. If any other DTC is present, go to applicable procedure for other DTCs. See DIAGNOSTIC TROUBLE CODE DEFINITIONS  . If no DTCs are set, trouble shooting is complete.
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