Got a p0128 a week or two ago when driving home about 1am just after it rained and was running my defroster. I assume that I had a thermostat issue back a ways and the only reason the code came up was that it was cold, rainy and the fans were always on with the defroster running.
I deleted the code when I got home and ran it in test mode for 3 days after reading this thread:
The code never came back.
it stayed between 67C-77C. Never below 67C except when warming up, stop and go traffic it would creep up to about 85C. Never saw it drop to 0C so I didnt think it was the sensor.
Checked out my thermostat and I lucked out it was the cheaper one 26.99 at autozone. Also picked up new intake gaskets, since I thought I would need to replace it.
HOWEVER!!! I was able to replace my thermostat/housing without removing anything. The hose clamps were a tad of a PITA, but I just need to get some long nose pliers. Lost about 1 gallon of coolant.
The old thermostat was closed when I removed it, so I'm not sure what exactly the problem was, maybe a spring issue? spring couldnt hold the coolant back?
After all said and done, tested it today, (the therm had NO temp markings, autozone site says 180*) and it kept going up until about 96*C and just kind of leveled off there. it only fluctuated between 96-102*C. 75mph on the hwy it stayed at 98-99*C. Seems right, since 100C is 212F.
My guess is that with the Sensor between cylinder #2 and #3 that it will be different than those who have the sensor in the housing itself where it may be closer to the actual opening temp?