1998 cadillac deville service engine soon code p0420 on all the time changed egr valve today have not driven yet autozone told me it could be bad cat converters

Who told you this was the part that is causing this stall?

The error code p0420 does not rise only due to faulty catalytic converter but may also rise due to one of following reason:

1. Faulty rear O2 sensor

2. Damaged O2 sensor wiring

3. Exhaust leak

The error code p0401 indicates faulty Engine Gas Recirculation (EGR) valve. The remedy to the problem is:

a. Replace EGR valve

b. Repair damaged wiring of EGR valve

c. Clean the EGR valve tubing

Follow the link for help with this fault code P1405 Ford DPF EGR Sensor Circuit High Voltage Detected

I suggest you replace the EGR valve position sensor.

well I'm a mechanic and you just told me what's wrong....
now all you have to do to fix it is test all parts listed replace the one (s) that fail test....

Not sure if you got an answer, however first thing you should have done was take it to AutoZone or someone with "diagnostic" computer module they can hook it up and tell you quickly. I know that my "soon" did the same thing and then "NOW" thinking it would go away like the fore mentioned "soon" and my TIMING chain busted. One thing you aren't told about when maintaining a fancy elite Cadillac or any other luxury car. The "maintenance" changes drastically. I had no idea that the timing chain needed adjusting every 60,000 miles. I had purchased my Cadi third hand and 105,000 miles. So at 140,000 miles it was way past due. Unfortunately, to replace the chain they had to remove the front of the engine. A $60 part cost $1300 to fix, but I did and she still IS the cat's meow.Point being, even though it was the early stages of computers/there is a diagnostic panel under the ashtray that can be used to trace these unforeseen problems.Hope your Cadillac is still going strong.

Subject:
Service Engine Soon (SES) Light On, Diagnostic Trouble Code (DTC) P0741, Torque Converter Clutch (TCC) Excessive Slip, Transaxle Chuggle, Slip/Surge (Inspect/Replace Internal Transaxle Components) #04-07-30-034 - (08/06/2004)
Models:
1998-2002 Cadillac Eldorado
1998-2004 Cadillac Seville
1999-2004 Cadillac DeVille
2000-2003 Oldsmobile Aurora
there is a write up on this about 3/4 page down.

http://www.cadillacforums.com/forums/cadillac-forum/t-107360.html

It should be either no Spark or no Fuel.
Possible causes: Out of gas, no oil pressure, faulty oil pressure switch, Bad fuel-pump, fuel-pump relay, needs tune-up, defective ignition control module, wrong air/fuel ratio, defective IAC valve or EGR valve.
Other causes: Wrong timing, bad compression, or clogged CAT.

cylinder 2 misfire po302

the light can be caused by any one or combination of over 200 trouble codes, go to any kragen or Autozone parts store and ask for a trouble code test, it is free, then record the code(s) they come up with and post them back on fixya