I keep getting this code. My manual says it means "catalyst system efficiency below threshold on left bank". I've replaced the O2 sensor in that manifold, code came back after a while. What is the big "coffee can" looking canister below left manifold? I also noticed what appears to ba another O2 sensor. It's on the cross-over after it switches sides, but before the two pipes merge together. I do have only "1" cat, correct? How can I get around this code?