The P0404 code on your 2019 Nissan Altima indicates the ECM has detected that EGR system operation is outside expected parameters. This range/performance code suggests the system is functioning but not within acceptable tolerances.
Understanding P0404
Unlike circuit codes that indicate electrical failures, P0404 is a rationality code. The ECM compares expected EGR behavior to actual operation. When the difference exceeds thresholds—whether position doesn't match commands or flow effects don't match expectations—this code sets.
Altima EGR System
The 2019 Altima uses the 2.5L QR25DE four-cylinder or available 2.0L variable compression turbo. Both have electronic EGR control. The system monitors valve position, commanded position, and downstream effects to verify proper operation.
Common Causes
A sluggish EGR valve that responds slowly or incompletely causes range issues. Carbon buildup affects valve movement precision. A failing position sensor may report inaccurate positions. Intake manifold carbon deposits can alter expected flow effects. Exhaust leaks near the EGR pickup affect system readings.
Diagnosis and Repair
Use Consult to monitor EGR valve response to commands. Compare position readings to actual valve movement. Test response time—the valve should move promptly when commanded. Inspect for carbon restricting movement. A valve that's slow or imprecise typically needs replacement or thorough cleaning.