Replaced cat. convertor at a muffler shop cost about $150.00 and still have code P0420 and now have more codes (C1214 and C1248) and abs light, service engine light and traction light is on. Took to dealership, they recommend cat. converter replacment for code P0420. Replace EBCM AND BPMV for codes C1214 and C1248.

SOURCE: replace egr valve

Your 2001 Buick has digital linear EGR valve, and should be able to be replaced and the SES light cleared. When the conditions are correct, while driving, the computer will open the EGR valve and watch inputs from the valve itself, as well as the MAP sensor, and Oxygen sensor to verify proper operation of the EGR valve. There is no reprogramming required to change an EGR valve on a 2001 model year Buick. General Motors may have a re-flash available to correct an EGR issue, and sometimes they recommend this re-flash with a new part. I checked my sources for you and could not find any TSB (Technical Service Bulletins) that are associated with EGR valve replacement, or re-flashing of the computer to correct erroneous EGR codes. Regrettably, it sounds like you have an issue with your mechanic, and POSSIBLY a misdiagnosis. Hope this helps!

SOURCE: 1995 Jeep YJ Exhaust / Muffler / Cat Conv on Hangers

use a jack to force the muffler into position, or you may have to move the hANGERS accordingly

SOURCE: 2002 Buick Century Code P0420

Sounds as if yoou are actually having a fuel system problem.You need to rreplace the fuel filter and then check the fuel pressure. You should have around 50 psi. If the fuel filter stops up it will cause the exhast to starve for fuel and can damage a catalyst converter. You can get the fuel filter for about 15 dollars at any local parts house.

SOURCE: had p0420 code on buick v-6. replaced cat.

A code P0420 may mean that one or more of the following has happened:

• Leaded fuel was used where unleaded was called for
• An oxygen sensor is not reading (functioning) properly
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
• Retarded spark timing
• The oxygen sensors in front and behind the converter are reporting too similar of readings.

SOURCE: 2004 Monte Carlo

The traction control and abs are both activated by the abs sensors located at all four wheels. It is common for those sensors to get covered in brake dust and get false or no signal. Also the wiring connections can become corroded over time. I am not sure what the codes are for but checking the sensors/wiring are where I would start.