P1408 — EGR Flow Out of Self-Test Range

Diagnostic algorithm

1. Retrieve and record freeze frame data and all stored/related DTCs. Note engine conditions when fault set.
2. Clear codes and attempt to re-run readiness/self-test while monitoring EGR commanded vs actual values and DPFE/EGR sensor readings with a capable scan tool.
3. Perform visual inspection of EGR valve, passages and ports for carbon build-up. Inspect vacuum lines, solenoids and electrical connectors for damage or corrosion.
4. With the engine warm and using live data, command EGR open/close (or perform active test) and observe EGR position sensor and DPFE response. If commanded movement doesn't change sensors, investigate valve/actuator and supply (vacuum or electric).
5. For vacuum EGR systems, apply hand vacuum to the EGR valve (engine off or as directed by procedure) to check mechanical operation and for leaks in vacuum lines/solenoids.
6. Perform a differential pressure test across the EGR circuit (if equipped) per manufacturer procedure, or use a smoke tester to find intake/exhaust leaks that affect readings.
7. If sensors (DPFE or EGR position) read implausible values or are slow to respond, inspect wiring with backprobe and measure voltages, grounds and sensor output. Repair as needed.
8. Remove and clean or replace a carbon-clogged EGR valve and clean passages. Re-test for correct flow and code disappearance.
9. If mechanical and electrical checks are good and flow still out of range, consider replacing the DPFE/EGR feedback sensor or EGR valve as indicated by tests.
10. After repairs, clear codes and verify via road test and re-run self-test to confirm the fault does not return.