What this code means
P0404 may mean the control module is commanding EGR valve movement, but the measured EGR position may not match the desired position as expected.
What the vehicle may do
- The vehicle may turn on the check engine light.
- The vehicle may display an emissions-related warning or reduced power message.
- Driveability may be normal, or the engine may feel limited depending on the active warning condition.
Possible fault areas
- Possible EGR valve control-side issue.
- Possible EGR position sensor feedback issue.
- Possible connector, wiring, or circuit issue.
- Possible EGR valve response issue.
Diagnostic path
Opening context
On a 2020 to 2025 Chevrolet Silverado 2500 with the 6.6 diesel, P0404 is an EGR open position performance code. In plain terms, the control module may be asking the EGR valve to move, but the valve position feedback may not be matching what was commanded. The truck may have an engine light, reduced power messaging, or an emissions-related warning, depending on what else is going on. Broadly, this can involve the EGR valve control side, the position sensor feedback, connector or circuit issues, or the valve not responding the way the module expects. Before getting deep into P0404, start with the basic system checks and follow a structured diagnostic approach so you are not chasing a symptom caused by another fault.
How the monitor thinks
This is a commanded-versus-actual EGR fault. The EGR output circuit is switched from ground to 12 V to activate the component, and the module watches whether the actual EGR position follows the desired position. The monitor is gated by operating conditions: the engine has to be running, ignition voltage has to be Greater than 11 V, and the related EGR circuit or position-sensor faults need to be clear. Once those conditions are met, the monitor runs continuously. P0404 sets when the commanded EGR state does not match the actual state for greater than 2.5 s.
First check with the key on
With the ignition On and the vehicle in Service Mode, check for related faults tied to the EGR valve circuit and the EGR position sensor. The expected result is no related DTC set. If a related code is present, check what that code means first instead of continuing straight into the P0404 performance test. If no related code is set, continue with the engine-running check.
Command the EGR valve and compare desired to actual
Start the engine. Using the scan tool control function, command the EGR valve up and down from 0 to 60% and back to 0%. While you do that, watch EGR Position Sensor against Desired EGR Position. The two values should stay Within 5% of each other. If the actual position does not stay within that range of the desired position, the supported repair path is replacement of the Q14 Exhaust Gas Recirculation Valve. If the two values stay within range, this test passes and the result is all OK for this part of the diagnostic path.
Verify the repair
After the repair is complete, verify the repair and confirm the code stays gone. Keep this separate from the command test: the command test checks valve response, and the repair verification confirms the fault no longer returns under the required conditions.
Check event data and clear any warning condition
With the ignition On and the vehicle in Service Mode, look at the Exhaust Gas Recirculation System Malfunction Warning Event Data. All of those event data parameters should display No. If any event data parameter displays Yes, operate the vehicle within the running conditions for the DTC identified by that event data parameter. The code that caused a driver message or speed limitation has to run and pass before the message or speed limitation will clear, and that may require driving the vehicle. If all event data parameters display No, the result is all OK.
Takeaway
The key on P0404 is to prove whether commanded EGR position and actual EGR position track each other. Check for related EGR faults first, command the valve through its range, compare desired to actual, then verify the repair and confirm any warning condition has cleared. For more diagnostic training, visit stepdiagnostics.com.
Final check
P0404 is often diagnosed by checking for related EGR faults first, then comparing commanded EGR position to actual EGR position during a scan tool command test.
For more guided automotive diagnostics, visit STEP Diagnostics.
