Why is trouble code P0128 still around after changing thermostat?

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Hey guys I'm a 2007 Mazda3 Hatchback owner looking for some help. I had codes P0126 and P0128 pop up. I like to do work myself to save money so I did my research and after diagnosing the problem, I realized I need to replace the Thermostat and the Engine Coolant Temperature Sensor. After proper installation, P0126 went away, but P0128 stuck around. I thought it might just need to be cleared so I cleared the code myself, only to find it kept popping back up. So I took it to my neighborhood mechanic and he told me that the monitors had to be reset at a Mazda dealer. The other day I took it to the Mazda dealer and they reset the monitors which cleared the trouble code... until about 45 miles later the code came back. I have run diagnostics to see if the engine is reaching the correct temp. After 5-10 minutes of idol I am reading 170-190 degrees near the place where the engine meets the thermostat and about 205 degrees at the top of the engine. I'm truly stuck here and late on my smog check for my vehicle registration. Any help would be appreciated, thank you!