I just got my 211K mile Honda Pilot back from my trusted local independent mechanic last week. Had timing belt, water pump, and tensioner done, plus spark plugs and valve adjustment. Valve adjustment was more labor than the timing belt, since the intake manifold comes off. Also had him throw on an ATF Temperature Sensor on the transmission, since it shifted late/high RPM when cold, and sometimes ignored 4th and 5th altogether until warmed-up. That was another hour due to: two engine mounts and pull a coolant line. Well the good news, purrs like a kitten, lifter tap sound is gone, and the temp sensor fixed the cold shift problem. So-So news is same fuel economy. The scare came near the end of running morning chores, involving one medium-short and 2 short trips, I got a CEL. Stopped in at my mechanic who was just opening shop, he read codes P0420 and P0430, and then cleared them, said come back if they come back. No other codes. Since then I've been reading up on these codes. Here's what I (think I've) learned. They mean front and rear Cat performance under par. It takes two failed runs in a row to throw the code. They happen more often in cold weather. They usually don't mean the sensors are bad, since the before-cat and after-cat O2 sensors have to agree in lock-step that the same amount of oxygen is going by. Usually these codes mean it is time for new Cats. Can be caused by intake leaks and diagnosed by fuel trims. Some more info: These short trips occurred in 5F-9F morning weather. The Cats are missing their Heat Shields, because they 1/2 rusted off and the flapping, buzzing remains were removed. Codes have not come back (yet). Vehicle is well-maintained and does not have any fuel delivery issues. I throw in a bottle of PAO injector cleaner twice a year. Vehicle did consume a few quarts of oil for about 30K last year until I figured it out and replaced the clogged PCV valve. Current oil consumption is about 1/2 quart per 7500 OCI. My thinking Working hypothesis #1- short trips, crazy cold and no heat shields mean Cats never got up to operating temp twice in a row when it moved from Open To Closed loop. Working hypothesis #2- My mechanic, while awesome is only human, and left an intake leak when he replaced the manifold (he did use new gaskets) Working hypothese #3- Cats are going bad. Obviously I am rooting for #1. I do have a cheap BAFX ELM327 and Torque, but I've heard there is a procedure to check fuel trims, it's not just done at idle in the driveway? What would be the easiest way to check this? Any other easy diagnostics? (that is, not pulling the exhaust and looking up the pipes). Any flaws in my reasoning, or other hypothesis?