I have a '00 Civic with 212k on it. I've had it for 11 years. It's been rock solid for the time I've had it, but as it ages some things begin popping up. A couple days ago I got a CEL for catalytic converter. While reading the code in my garage I noticed the motor's temp spiked slightly above normal and came back down. Yesterday I got stuck in traffic coming home from the office and the temp started rising again. I turned the heat on to be safe and it leveled off. When I'm driving the temp is fine. I got home and listened for the fan to kick on and it did, although I may have taken longer than I expected. I am familiar with how it cycles "normally" (which may or may not be correct). It was only 70 degrees yesterday here, so it wasn't a particularly stressful day for the cooling system. I had the radiator replaced two months ago and it looks like I've lost some coolant from the overflow tank. I noticed it previously because I thought it was overfilled but concluded it wasn't a big deal. I'm wondering if the P0420 code plus the sudden tendency to want to overheat is a sign my head gasket is done. Any thoughts? I'm watching the coolant level and will try to log some miles this weekend. No visible smoke out the tailpipe. Oil on the dipstick didn't look out of the ordinary. Add this to the broken a/c, the need for a routine timing belt/water pump service, and the stripped oil drain plug thanks to some untrained "mechanic" changing the oil and it might be time to part ways with the Civic.