2004 MAZDA 3 ECT SENSOR - P0128 CODE

Electrical problem
2004 Mazda 3 4 cyl Two Wheel Drive Manual 58000 miles

Until my engine is warm my car operates perfectly. Once the car is warm and goes from open-loop mode to closed-loop mode the performance becomes inconsistent (hesitation and varying power output). I believe this is due to a sensor and/or wiring problem. I have read that many people have issues with the MAF sensor wiring but I have been unable to find any broken wires and I have chased down the wiring with a multimeter (also cleaned the MAF/IAT and did fuel injection system service). One day I cleaned the PCM wire harnesses and the car drove perfectly for almost two days but since then no amount of cleaning makes a difference.

Just recently, I have been getting the P0128 code which suggests the thermostat or ECT sensor is bad. There is a TSB out for this problem but I recently had my PCM updated and the dealer said this update would have solved the problem if the PCM was the cause. The temperature gauge in my car never moves a millimeter once it is warm so I do not think the thermostat is the problem. I have read that the P0128 code is based on a calculation that includes input from the MAF and IAT sensors which seem to be the more likely suspects. Does anyone have any advise before I start replacing parts?

I have read that this P0128 code does not let the Mazda3 get out of open-loop (start-up) mode. I am not sure if this statement was accurate but the author was confident that this trouble code would impact engine performance. My car has not behaved any differently since the CEL first came on about six weeks ago due to the P0128 code. My problems are intermittent, only happen when accelerating, go away for hours (sometimes day), but always come back. When my car is experiencing the problems the gas mileage is poor (21 vs 28 mpg) and there is excessive carbon build-up on the muffler tip. I am not sure if my problems are related to the P0128 code but I sure hope so.