How to fix P0128 and P0420 on chevy aveo 2010?

I have chevy Aveo 2010 LT. Recently I got the P0128 and P0420 codes
about at the same time. The temperature gauge was at the quarter for more
than a month (when I drive the car) and never reached the operating
temperature even after hours of driving. Yesterday I took the car again on the
highway and the gauge indicator dropped to C and never came back up
again. I checked the ECT sensor mounted directly above the thermostat
(Sensor 1) and when it is disconnected the voltage across its harness leads
is 12V (when the switch is ON), and its resistance ~2K ohm after driving
about 20 min. I also checked the ECT sensor on the thermostat housing
(Sensor 2), the voltage across its harness leads is 5V (when the switch is
ON), and its resistance is 8K ohm when the car is cold. I was not able to
check the last sensor at the bottom of the radiator (Sensor 3). I don't know if
the voltage across Sensor 1 is normal or not. Are those the ECT sensors in
the car? What is the wiring diagram? How can I fix the problem (at least the
P0128)? The mechanic asked for $400 to fix it and this very expensive to
me. I appreciate any help, thank you very much.