Because you did not have a faulty EGR valve. You misdiagnosed the problem. Start over and remember the code only tells you what part is being effected not necessarily the defective part. This is assuming you reset the light or drove the car enough cycles for the light to reset. If not then reset the light and see if it comes back on.
Check for a vacuum leak.
check your manuel
Get a code read out from your engine computer and it will tell you what is wrong.
Check www.autozone.com, they have a Repair Info section that covers Buick LeSabre with this info.
way to go genius you screwed up the car
Check the belts. The most common cause of squealing is a worn or loose belt on the front of the engine.
Most any repair shop will do it for a fee. If you are looking for free, try Autozone.
If you are referring to resetting the Check Engine Light then you will need to have someone with a scanner to do it. If you are referring to reprogramming the Engine Control Module then the dealer will do it.
To repair a seat heater on a 2004 Buick LeSabre, disconnect the connector and check for voltage. If there is no voltage, replace the switch. If there is voltage, check for an open circuit.
There are pages and pages of things that can cause a check engine light. You need to have it checked with a scantool. Then the code can be diagnosed and repaired.
1997, 98 and 99 lesabres are supposed to bigger inside so check with a dealer or wrecking yard.
Owner's manual.