89 Chevy C1500 Trouble Code 13 " Again"

Ok guys here is my Dilemma, I have a 1989 Chevy C1500 w/ 350, T.B.I, Automatic Trans, Mileage= High, that keeps throwing the Trouble Code 13.. I can crank the truck and drive it and it runs great BUT as soon as the engine gets warm(5 -10 miles) the Check Engine Light comes on and I can tell a noticeable difference in power (loss).. I have to press a little more on the gas pedal to maintain my current speed.. As you know a Code 13 indicates a faulty O2 Sensor, I replaced it with NO LUCK.. Could I have bought a faulty O2 Sensor? I have heard of Brand New P.O.S Before.. The old O2 sensor was white on the end if that helps any and I checked the voltage of the wire coming from the ecm with the switch in the on (not running) position and I got a voltage of 1.5volts if that helps any.. Oh and all this started all of a sudden after I done a complete tune-up on the engine. New Plugs,Wires,Cap,Button,PCV,Air Filter,Oil and Filter, New Thermostat and Gasket.. Any help would be GREATLY APPRECIATED and I am willing to try anything at this point. Thanks for any Help.

The Oxygen sensor has an internal heater to warm it up. Use a dvom and check that you have power to the sensor. If you used one where you have to install the plug these are a big problem area.

Remember, 89 vehicles did not have heated 02 sensors.

I missed the year Max is correct. I've had a lot of bad Oxygen sensors come in when I had a garage most had wires that needed cut and attached. The connectors some I found disconnected. I think the connectors were the wrong size. We threw them in the trash and purchased sensors with the plugs connected. This generally corrected the code.
If you used a 180 degree thermostat you may not be getting hot enough for the system to get into the proper loop. Is your temperature gauge getting as hot as it used to before it was changed.

Guys, the OP moved this post to the C/K forum and has been getting feedback there.