When Monitored: The engine running. During cold start.
Set Condition: The PCM predicts a coolant temperature value that it
actual coolant temperature. If the two coolant temperature values are
(18°F) of each other an error is detected. Two Trip Fault.
Something is either taking TOO long to warm up, or it's warming up too quickly. Possibly low fluid, fan problems, incorrect thermostat, broken thermostat, etc.
I got the P0128 code twice this winter, both times it happened on very cold days. I just reset the PCM and the code went away. It has not come back since the last time which was a couple of months ago.
Try resetting the PCM. If the code reappears, then start checking the thermostat, fan, etc.
It means you have one of the rare female thermostats. Much like women they are sometimes irrational and can have severe mood swings. Does this code come up every 28 days or so? If so, then there is your answer. It is a hormone thing. Putting birth control pills in your coolant can help but it’s easier to switch to a male thermostat.