Can someone tell me what setting a P0404 and P0401, with CAT removal will actually hurt? What does this actually do to the EGR functionality?

it wont do anything, its just a oode that the computer picks up because its not seeing what it thinks it should, and will have no adverse affects on anything ( the code that is )

So the EGR keeps working as it is supposed to? It doesnt cause it to pump more or less crap into the motor?

The CAT caused more back pressure to the exhaust system due to the CAT's restrictive nature to flow. This caused more pressure to the EGR valve therefore more exhaust will flow through the EGR when opened. If I'm off base someone please chime in.

Ya, I was curious about that too, if its a silent code stored in the box, and really doesnt change anything its no biggy to me. Thats how I read into his question. I know some things that throw codes reduce power, and that would defeat the purpose of cat/egr mods if a code reduces power.

It will illuminate the MIL on CA trucks, but not FED trucks...

it never set a light on my truck.. I am just curious how it affects the EGR?? or if it is reducing power some how. Didnt seem to.

the egr should remain working the same way, if anything you would be blowing more crap out since the cat isnt pushing it back towards the egr. it sets the code because it thinks it should see x amount of back pressure and when there isnt a cat it doesnt see that information so it throws a code. the next thing for you to do if interested is get the egr disconnect set up then you wont have the egr and no codes !!

IF the 401 and 404 codes are set, the EGR valve will not be operated by the ECM. It is not clear if the ECM does anything else based on the codes being set. (timing, fueling, boost, VVT setting, or whatever)

Can someone tell me what setting a P0404 and P0401, with CAT removal will actually hurt? What does this actually do to the EGR functionality?The following is taken directly from GM service information:
(you did not say if you are in California, so here is the "Federal" info)

P0401:Action Taken When the DTC Sets - Federal (http://service.gm.com/servlets/BlobShtml?ShtmlFile=1488410&psdid=1058&evc=sm#ss6-1488410)

The control module stores the DTC information into memory when the diagnostic runs and fails.
The MIL will not illuminate.
The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.
The driver information center (DIC), if equipped, may display a message.
P0404:Action Taken When the DTC Sets - Federal (http://service.gm.com/servlets/BlobShtml?ShtmlFile=1485693&psdid=1058&evc=sm#ss6-1485693)

The control module stores the DTC information into memory when the diagnostic runs and fails.
The malfunction indicator lamp (MIL) will not illuminate.
The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records.
The driver information center (DIC), if equipped, may display a message.
As you can see, there will be no change in engine operation as a result of the failures. The first code will set as a result of Mass Airflow irregularities caused by a blocked-off valve or exhaust modifications. The second code will set as a result of improper electrical feedback caused by disconnecting the EGR wire.
The PCM still continues to attempt to operate the EGR valve in spite of the failure codes.

(further explanation) The PCM does not know the term back pressure, it only compares intake air flow with known parameters (such as when the EGR is opened and closed). If exhaust back pressure is reduced, the PCM sees a Mass Airflow that is outside the anticipated range and sets a trouble code.
Ed

I know from observing the EGR that the ECM will not run the EGR valve if it does not pass the function test (flow test). Been watching it a lot while I developed the Stick. If you doubt me, check it yourself on a truck.