Rich O'Rourke

Hi everyone, I replaced a cat since the muffler clamp broke and the weight of the exhaust assembly ripped open,the back of the cat. I replaced it with a National Auto one that looks just like the chevy version.I put 540 miles on it and the P0420 code came on. I replaced the temp sensor on the engine block, thermostat and dexcool, the engine did run in temp specs at that point, before it was 10 degrees too high. I replaced the spark plugs and wires with official GM oem. I even replaced the two coils( one coil goes to a pair of plugs,other coil goes to other pair of plugs) I took off the top engine cover shield and replaced the pressure control valve near the fuel injectors, but did not replace the fuel injectors. I replaced the oxygen sensors with Bosche Oxygen sensors , one in the manifold, theother after the cat. When i make a change and zero the codes, the P0420 pops up again, sometimes after 7 start cycles, sometimes after only 18 miles and stays on. The spark plugs were clean so I do not have excess oil consumption. One member suggested putting a delco oxygen sensor after the cat, and stated that the bosche sensors were too rapid a response. He said the delco sensor would be more lazy like the original sensor that came with the car. i have not checked to see if the spark timing is retarded. How do I check the spark timing? I just bought an engine tester from Harbor freight and it gives live data when the engine is running and i can see the wave form of the sensor in the manifold, it goes up and down, but the rear oxygen sensor is not supported for the code reader from harbor freight to pick up. How can I check the spark timing to see if it is retarded? I have a timing light but do not know how to hook it up. My car has wonderful acceleration now and gets 28 MPG. and that is with 85000 miles on it. Do you think I should put new or refurbished fuel injectors it it? Or should I try one of those delco oxygen sensors after the cat? I would appreciate any help anyone can give me on this one!!! Also, I did not feel any leaks around the exhaust connections or cat. How can i check for leaks around the manifold gasket? As you can see I have been really trying to fix this problem and the changes I made did improve the performance and mpg of the vehicle. But the code still comes up and will fail inspection and the cold weather is creeping in now. Can someone help me with this? Thanks, Rich

MDTAHOE

Replace the cat with a better quality. 99% of the time PO420 is a bad cat. New does not mean good. More that 1/2 of the aftermarket cats installed do not work well enough to keep the CEL off. Read the emissions label on the hood. Does it say the car has California emissions? If so you must use a cat that meets California standards.