Before driving from CT to UT, I finally fixed my coolant leak due to bad intake gaskets. I had been leaking progressively worse for months, and burning coolant at a pretty alarming rate until I fixed it in January. I subsequently replaced the spark plugs and changed the oil of course. Everything went fine and the drive was smooth, 35mpg across the country. Just last week she decided to throw a P0420. Drivability is fine, though mileage seems to be down to around 31mpg on the highway. The exhaust is very new all around, including the cat.
Could the burning of coolant have fouled the sensor(s)? I monitored the voltage while idling. Sensor 1 fluctuated up and down from around .1 to .8 mV. Sensor 2 stayed relatively steady between .3 and .5 mV. RPMs weren't as steady as they used to be, but not bad. I'm highly doubting a leak due to how new the exhaust system is, does it make sense that burning coolant for so long has fouled something? And if so, how do I check? What will a coolant fouled O2 sensor look like?
If it's relevant at all, I also had to replace my water pump after I got here. I really did a number on the coolant system running it low so many times. If only my coolant level sensor worked...