[hatzie]

I googled "GM P0404 Code set Criteria"
This is what I found in one of the first page links.

" In order to know if your EGR’s valve is operating properly, the EGR valve’s feedback system communicates with your Power Control Module (PCM) to let it know if your valve is open, closed, or partially closed at any given time. When P0404 is triggered, it means that the PCM has been told that the valve is performing in a way that is outside of its specified parameters. More specifically, the PCM has been told that the valve is closed when it should be open, or vice-versa."

Bad EGR flow can cause spark knock from higher than normal combustion temps. When this happens the PCM will pull, or retard, ignition timing cutting your performance.

If the PCM is pulling timing because the EGR is sticking and the flow isn't keeping the combustion temps low enough it'll be low on power.

You really need a tool to view items like O2 sensors, Long & Short term fuel trims, knock sensor data, EGR data, and ignition timing advance or retard to properly diagnose this and to prove your fix was successful.

In the good old days the remedy for spark knock was higher octane fuel. You could keep a jug of high test fuel in the bed and run her down to excessively near empty then fill up with some high test fuel and see if she seems to be happier with you. By no means is this a foolproof test but it might tell you something about what's going on.