Faulty ECT sensor?
 

I have an 05 Mazda3 w/90k mi. The check engine light came on and I took it to Mazda, they said I needed a new thermostat. But the thermostat seems to be working fine, its always in the middle. Then I took it to Autozone and they got code P0128 Faulty sensor. Then I heard that some 3's were getting this faulty sensor and they did a reflash and reset the computer and its fine. Is there anyway I can reflash myself or do I need to take it to Mazda to do this? Or should I replace the sensor and/or thermostat? The car is running fine. No problems at all. I hate to replace something that isn't broken. Please help!

You can drive on a flat tire too but its not a good idea... if the cars running cold the compuiter will send in more fuel to help it warm up...that will wash the oil from cylinder walls wearing engine rings and bearings..it will also ruin cat converter and egr and o2 sensors..

That sensor shouldn't be expensive, it really depends on the location. Maybe you can do it yourself if you gots the tools. I am sure the sensor is on the line of the Coolant Lines with the thermostat. Might as well change both at once with OEM unit..