My mazda's check engine light is on and the readings are P0400 and P0421. Can a carbon build-up in EGR valve cause a rough idling and set a P0421 for cat. efficiency? Also lights dim when i use power windows? Please help

 

If the EGR valve has been sticking for some time and causing a rough idle and missfire then it could definitely cause the P0421 cat efficiency code. Engine missfires are very, very hard on cat converters. That is why the CEL flashes during a bad missfire to inform you that something needs to be done right away.
Lights dim with all of the windows or only one?

 

Thank you very much. The CEL ligth is off and it runs good now, although i didn't do anything. I think it's just a matter of time before the the CEL returns. I read your comments and i think i need to clean egr valve as soon as possible. And as for warm up cat., i'll change if the CEL returns after i clean egr. Lights seem to dim with every window.

 

A slight dimming of the lights is not too abnormal as the window motors draw a bit of amperage. Can also be agravated by dry window run channels that cause excessive drag on the window and make the motor work even harder. Make sure though that the battery and charging system are working normally as this could also be an indication of weak battery or low alternator output.