Low Effiency Catalyst

[rtobrian]

I have a 2002 Grand Am 3.4 with 100k miles on it. I had the catalytic converter replaced about 3 months ago. I also replaced the rear oxygen sensor. The upstream oxygen sensor was replaced about a year and a half ago. The service engine soon light came on a couple weeks ago saying low effiency catalyst. I immediately took it back to the muffler shop and they said the catalytic converter was fine that is was something upstream. I found a crack in the upper manifold and sealed it with jb weld and the light went off and the car ran good. A week later the light came back on with the same code. I took carb/intake cleaner and sprayed the engine down and the car didn't react so I don't think it's a vacuum leak. I'm up for any ideas you guys may have...

[Playamade]

Maybe a clogged muffler?

[rtobrian]

I changed the upper intake manifold with a brand new one. I also checked and replaced some breather hoses. I changed both O2 sensors again as well. The service engine soon light came on again with the same code. Since then I've retorqued the upper intake bolts, replaced ignition module and coils, pcv valve, purge solenoid and valve, air filter and oil, fuel pressure regulator, air temp sensor, knock sensor, coolant temp sensor, map sensor, EGR Valve, maf sensor, fuel cap, alternator, coolant hoses, and cleaned the throttle body and iac. The plugs, wires, fuel filter, 180 degree thermostat, lower intake gaskets, water pump, and cat were changed within the last six months. I also ran fuel injector cleaner in it. I reset the ses light and she drove the car to Tennesse and back without any problems. Then just the other day here comes that stupid light again. I'm at my wits end with this car! I don't know what else it could be?