What this code means
P0404 means the EGR valve position may not be matching what the engine control module expects when it commands the valve.
What the vehicle may do
- The vehicle may display a Service Emission System message.
- The vehicle may display a Service Exhaust Fluid System message.
- A message or speed limitation may remain until the related fault monitor runs and passes.
Possible fault areas
- Possible EGR valve concern.
- Possible EGR position feedback concern.
- Possible EGR control circuit, connector, or harness concern.
- Possible engine control module concern after the circuit and valve diagnostic path has been completed.
Diagnostic path
Opening context
For P0404 on this Silverado 2500 diesel, think of it as an EGR position performance problem. The module is commanding the EGR valve, but the actual valve position may not be matching what was commanded. The truck may turn on a Service Emission System or Service Exhaust Fluid System message, and depending on what has set, a message or speed limitation may not clear until the fault runs and passes. Broadly, the possible fault areas can include the EGR valve itself, the EGR position feedback, the valve control circuits, connector or harness issues, and only after the circuit and valve path checks out, the engine control module. If other codes are present, check what they mean first, but keep this diagnosis focused on P0404.
How the monitor thinks
Start with the basic system checks, then follow a structured diagnostic approach. The EGR valve redirects exhaust back into the intake to help reduce NOx, and on this system the valve is moved by a bidirectional DC motor. The engine control module controls the valve through output circuits that switch from ground to 12 V to activate the component. For this monitor, treat the run conditions as gates: the engine has to be running, ignition voltage has to be Greater than 11 V, and once those gates are met, the monitor runs continuously. P0404 sets when the EGR valve commanded state does not match the actual state for greater than 2.5 s.
Initial scan-tool verification
With the ignition on and the vehicle in service mode, look at the EGR Position Sensor parameter. It needs to be 0.25 to 4.75 V. If that parameter is outside that range, pause this path and diagnose the position sensor range problem first. If it is in range, operate the vehicle so the monitor can run, or reproduce the captured operating conditions. Then verify that P0404 does not set. If it stays gone, the result is all OK for this path. If P0404 resets, move into circuit testing.
Control circuit terminal 1 testing
For circuit testing, turn the ignition, vehicle, and all vehicle systems off. Disconnect the EGR valve electrical connector, then disconnect the engine control module electrical connector. Turn the ignition back on with the vehicle in service mode. At control circuit terminal 1 on the component harness, test to ground for less than 1 V. If you see 1 V or greater, repair the short to voltage on that circuit. If voltage is less than 1 V, test terminal 1 to ground for infinite resistance. If resistance is less than infinite resistance, repair the short to ground. If that passes, test between control circuit terminal 1 at the component harness and terminal 95 X2 at the control module harness. That circuit needs less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance in the circuit.
Control circuit terminal 5 testing
Next, stay in the same test setup and move to control circuit terminal 5 at the component harness. Test terminal 5 to ground for less than 1 V. If it is 1 V or greater, repair the short to voltage. If voltage is less than 1 V, test terminal 5 to ground for infinite resistance. If resistance is less than infinite resistance, repair the short to ground. If that passes, test between control circuit terminal 5 at the component harness and terminal 94 X2 at the control module harness. That circuit needs less than 2 Ω. If it is 2 Ω or greater, repair the open or high resistance in that circuit.
Repair decision and verification
If both control circuit paths pass those checks, the diagnostic call is to replace the Q14 Exhaust Gas Recirculation Valve. After that, connect the engine control module electrical connector, operate the vehicle under the monitor conditions or the captured conditions, and verify P0404 does not set. If P0404 sets again after the EGR valve replacement and the verification drive, the diagnostic call is to replace the K20 Engine Control Module. If it does not set, the result is all OK. Keep verification separate from the circuit testing: after completing the repair, verify the repair and confirm the code stays gone.
Final event-data check and takeaway
One more check: with the ignition on and the vehicle in service mode, look at the scan tool event data for the EGR system malfunction warning. All of those parameters should display No. If any parameter displays Yes, operate the vehicle within the monitor conditions for the code identified by that event-data parameter. The takeaway is simple: prove the position signal is in range first, make P0404 fail under the right conditions, then isolate the two EGR control circuits before condemning the valve or module. For more diagnostic training, visit stepdiagnostics.com.
Final check
Diagnose P0404 by verifying the EGR position signal, reproducing the monitor conditions, then testing the EGR valve control circuits before making the repair decision.
For more guided automotive diagnostics, visit STEP Diagnostics.
