My car shows 190F during cruise but shows 200F - 216F before fans kick in when idle.(live data on reader) How on earth is it showing less than open temp of 195F of a brand new thermostat and...........Coolant Temp Sensor! All GM at that! This is causing it to fuel more it seems and then kicks out a P0420 code.(New cat 1.5 years ago) I/M says Cat test "OK". At idle and 0 speed O2 senor data is steady but when I move it behaves like the front O2 sensor. Very perplexing for sure. Any and all help will be appreciated.

 

the bank one cat isn't working. the 02 will switch slow at ide and fast underload....the fact that the rear switches fast means the cat is done.

when driving you will get more cooling effect as the waterpump is moving faster and more air is flowing through the rad vs idle and no fan.

 

Since the sensor is in the head shouldn't the coolant be at 195F minimum? That is what is puzzling to me. It shouldn't be at only 190F since the thermostat would be closed and no coolant would circulate into the radiator during cruise. I agree that the cat maybe toast now since it is being mis-fueled. It's been a year and half since I replaced the cat. I might try the old cat to see if it was OK before I changed it. I now have a little over 200K miles on it. This has been hard to wrap my head around.

 

most likely the thermostat begins to open at 80*c 176 f and is fully open at 195.