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.