CEL in cold weather

My truck is a 1999 Tacoma 2.7L 4x4 3Rz. It has over 160K miles.
Installed new front and rear O2 sensors and they check out fine using a good OBII scanner. Before I installed the 02
sensors the codes indicated the sensors were bad so I replaced them. The new ones now indicate they work fine. The one
code I do get now is P0420. The check engine light only displays when it is pretty cold outside and even after I have
driven it a long time and it is warmed up completely it stays on. Lately it is warmer outside and it is not displaying
at all and it is running a bit smoother. All fluids are fine too as well as the gas cap. Spark plugs are good. No
noises. These 02 sensors were bought from my Toyota dealer parts department and are Denso. I installed these 02
sensors a few months ago while the weather was warm and have not had any engine lights or any other issues until it
just began to get cold outside so my thinking is that the sensors are fine. Also, I know how easy it is to damage 02's
so I was careful not to bang them at all or drop them during installation. I do not visibly see an exhaust leak or
feel one. According to this code it could be the coolant temperature sensor. Has anyone had this issue? Could I still
have a small exhaust leak? I should would like to figure out for sure if my catalyst is failing or not since a good
one cost some $$.