I know this is basic, well-known emissions test trick, so sorry if you already know about this :) . But have you tried driving it at highway speeds for like 45 minutes prior to testing? I only bring it up because my friend has an XJ with 2.8L V6 and he fails every two years for the exact same thing: "failed the tail pipe emissions inspection". Then he takes it on the highway for a bit, comes back, and passes. I don't know if it's legit, or just coincidence.