I HAVE AN 04 LLY AT WORK RIGHT NOW WITH P0401 AN P0404 AND IT ALSO HAS THE MAGNAFLOW STRAIGHT THROUGH SYSTEM. GM TAC TOLD ME NOT TO ATTEMPT REPAIR WITHOUT THE STOCK EXHAUST SYSTEM. I WAS TOLD BY CUSTOMER THAT HE RAN THE EXHAUST FOR A LONG TIME BEFORE THE SES CAME ON.(NOT SURE HOW LONG). WE REPLACED THE EGR BUT THE CODES DID RETURN. SO I TOLD HIM BEFORE WE COULD GO ANY FURTHER THET WE NEED TO REINSTALL THE FACTORY STUFF AND SEE IF THE CODES RETURN. KENNEDY HAVE YOU BY CHANCE CHECKED TO SEE HOW MUCH BACK PSI THE FACTORY EXHAUST MAKES? IT SEEMS TO ME THAT WHEN THE CODES SET THE EGR IS AT A VERY OPEN POSITION(LIKE IT MAY NEED BACK PSI TO HELP IT CLOSE FASTER)AND WHEN YOU CHECK IT IT IS OK.WOULD LEAVING THE CAT ON CAUSE TOO MUCH RESTRICTION FOR CHIPS? WHAT WOULD HAPPEN IF SOMEONE USED AN EXHAUST BRAKE? KENNEDY YOU SEEM TO HAVE THE RESOURCES TO EVALUATE THIS. COULD A MUFFLER BE DESIGNED TO GIVE ENOUGH BACK PSI TO MAKE THE EGR HAPPY? YET BREATH ENOUGH TO MAKE GOOD POWER. IT ALSO SEEMS THAT THE CODES ARE SETTING AT IDLE,AS IF WHEN THE SELF TEST OCCURS.LET ME KNOW HOW THE EGR DISABLE WORKS OUT THAT YOUR WORKING ON. THE EGR USES A WORM GEAR TO OPEN BUT RELIES ON SRING TENSION TO CLOSE IT. COULD A BIT STIFFER SPRING ON THE VALVE BE THE ANSWER?(MAYBE COMPENSATING FOR THE LOST BACK PRESSURE).THE END CAP ON THE EGR LOOKS LIKE IT CAN BE REMOVED TO GET AT THE SPRING,DON'T IMAGINE IT SHOULD BE THAT MUCH STIFFER JUST ENOUGH TO COMPESATE.WHAT DO YOU THINK JOHN?