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DTC P0401/16785 - EGR Flow Insufficient Detected

NOTE: DTC is set when coolant temperature is more than 162°F (72°C), open idle switch, throttle angle less than 42.5 degrees, vehicle speed 44-66 MPH, EGR duty cycle more than 50 percent, engine speed 1500-3300 RPM, and engine load 23-60 percent, with EGR temperature signal less than 122°F (50°C) for more than 34 seconds.
  1. Inspect EGR vacuum hose for kinks, disconnection from EGR valve and leaks. Repair vacuum hose as necessary. If vacuum hose was not repaired, go to next step. If vacuum hose was repaired, erase DTC memory and test drive vehicle.
  2. Inspect vacuum hose between EGR vacuum regulator solenoid valve and EGR valve for disconnection and leaks. Repair vacuum hose as necessary. If vacuum hose was not repaired, go to next step. If vacuum hose was repaired, erase DTC memory and test drive vehicle.
  3. Disconnect EGR vacuum regulator solenoid valve electrical connector. Using a DVOM, measure resistance between EGR vacuum regulator solenoid valve terminals. Resistance should be 25-35 ohms. If resistance is to specification, go to next step. If resistance is not to specification, replace EGR vacuum regulator solenoid valve. Erase DTC memory and test drive vehicle.
  4. With ignition on, connect VAG 1527B voltage tester between EGR vacuum regulator solenoid valve harness connector terminal No. 1 and ground. Using VAG 1551 scan tool, use OUTPUT DIAGNOSTIC TEST mode, function 03 to trigger fuel pump relay. Voltage tester should light. If voltage tester does not light, go to next step. If voltage tester lights, go to step 8).
  5. Check fuse for EGR vacuum regulator solenoid valve. If fuse is okay, go to next step. If fuse was replaced, erase DTC memory and test drive vehicle.
  6. Using a DVOM, check resistance between EGR vacuum regulator solenoid valve harness connector terminal No. 1 and fuse. See WIRING DIAGRAMS  section. Maximum resistance is one ohm. If resistance is not within specification, go to next step. If resistance is within specification, check fuel pump relay and relay triggering. See SYSTEM/COMPONENT TESTS article. Repair fuel pump relay circuit or replace fuel pump relay as necessary. Erase DTC memory and test drive vehicle. If fuel pump relay triggering is okay, go to next step.
  7. Repair wiring as necessary. Erase DTC memory and test drive vehicle. If wiring is okay, check fuel pump relay and relay triggering. See SYSTEM/COMPONENT TESTS article. Go to next step.
  8. With ignition on, connect VAG 1527B voltage tester between EGR vacuum regulator solenoid valve harness connector terminals No. 1 and 2. Using VAG 1551 scan tool, use OUTPUT DIAGNOSTIC TEST mode, function 03 to trigger EGR vacuum regulator solenoid valve. Voltage tester should flash. If voltage tester flashes, circuit is okay. Go to step 13). If voltage tester lights up constantly or does not flash, go to next step.
  9. Turn ignition off. Disconnect Powertrain Control Module (PCM) connectors. Connect VAG 1598/19 test box to PCM wiring harness. Terminal assignments of test box are identical to PCM connectors "A", "B", "C", "D" and "E".
  10. If voltage tester had lit up constantly, go to step 13). If voltage tester did not flash, using DVOM, check resistance between EGR vacuum regulator solenoid valve harness connector terminal No. 2 and test box socket E1. Maximum resistance is one ohm. If resistance is not within specification, go to next step. If resistance is within specification, circuit resistance is okay. Go to step 12).
  11. Repair open circuit as necessary. Erase DTC memory and test drive vehicle. If open circuit cannot be located, replace faulty PCM. Code new PCM and test drive vehicle. See PCM CODING  this article.
  12. Using DVOM, check for short to B+ between EGR vacuum regulator solenoid valve harness connector terminal No. 2 and test box socket E1. If short to B+ is present, repair as necessary. Erase DTC memory and test drive vehicle. If short to B+ cannot be located, go to step 14).
  13. Using DVOM, check for a short to ground between EGR vacuum regulator solenoid valve harness connector terminal No. 2 and test box socket E1. If a short to ground is not present, circuit is okay. Go to step 14). If a short to ground is present, repair as necessary. Erase DTC memory and test drive vehicle.
  14. Inspect EGR vacuum hose for kinks, disconnection from EGR valve and leaks. Repair vacuum hose as necessary. If vacuum hose was not repaired, go to next step. If vacuum hose was repaired, erase DTC memory and test drive vehicle.
  15. Ensure engine oil temperature is a minimum of 122°F (50°C). Start engine and let it idle. Disconnect vacuum hose at EGR valve. Connect a hand-held vacuum pump to EGR valve. Apply vacuum to EGR valve. Idle quality should deteriorate. If idle quality deteriorates, EGR system is okay. Erase DTC memory and test drive vehicle.
  16. If idle quality does not deteriorate, replace EGR valve if necessary. Erase DTC memory and test drive vehicle.
  17. After performing entire test procedure, if all electrical circuits test good and EGR valve is mechanically functional, and DTC returns, replace faulty PCM. Code new PCM and test drive vehicle. See PCM CODING in this article.
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