Well after 212k, the CEL came on and stayed on for the first time in my Civic's 13 years. I do believe this code came on about 1.5 years ago after getting an oil change. I turned around and had the mechanic pull it, and he reset it to see if it would pop again. Didn't come back until now. Pulled the code with Torque and it was P0420. I also ran the app's "tests" and it said I failed the "Rich to Lean Sensor Switch Time". Minimum value is apparently 2300 and I registered 2119. I've done some digging around Civic forums already for some things to consider. Here is what I think are relevant details so far. I changed my spark plugs and wires about 2 weeks ago. The plugs I pulled didn't look bad, I still have them, so maybe I'll take a closer look. After replacement, I noticed the somewhat rough idle I would get near the end of my 40+ mile one way commute went away. Other than that rough idle, the car has been performing fine since plug replacement. Same as always really. It seems a lot of P0420 codes pop after plugs/wires/distributor replacement. I've probably put about 1200 miles on it since changing the plugs. I got gas last night, filling a nearly empty tank from a Shell I rarely use, but was convenient for where I was stopping on my way home. Gas mileage for the previous tank was on the high end of my usual range. I had a ~6 mile drive afterward going 40-65. A couple stop lights. The code came on this morning about 2-3 miles into my morning commute going highway speed. Performance for my commute today was normal. Fuel consumption so far seems OK, although I did idle for a while in the garage which may throw this tank off a bit. One thing that did slightly concern me was a temperature spike I noticed while sitting in my car idling and playing with Torque tonight. I looked up from Torque to notice the gauge was about 1/8th of the way up the dial higher than normal (if that makes sense), it quickly went back down to normal almost as soon as I noticed it. I popped the hood and thought my overflow tank looked a bit lower than I remembered (which I remember since the place that replaced the radiator had filled it higher than I expected it to be). I'll be keeping an eye on it. When idling I did not notice any smoke out of the tailpipe. Exhaust note sounds fine, so I don't suspect a leak. Oil did not look cloudy on the dipstick. These are the original O2 sensors and catalytic converter. So it could just be time I guess. I just read about way too many "P0420 chases" on various forums, with guys replacing cats, sensors, plugs, and who knows what else trying to clear this code. So I'd like to have narrowed things down before dropping it with the mechanic. Torque seemed to show me the waves of the O2 sensors, which I'll check tomorrow after continuing my crash course in O2 sensors to see what they should look like. Anything else relatively easy to check? Best case I won't be able to get it into a mechanic until next Friday. Too much going on at the office. In the mean time, I might test drive some new cars. O2 sensors and/or hunting down the cause of the code might cross my repair bill line when added to the timing belt service I need done, broken a/c that needs to be fixed, and new oil pan I need. Appreciate the wisdom as always.