PO128 Engine Warning Light Indicator--HELP!

I've run into this code ("PO128") twice in 3 months, but I haven't found a solution posted here. The code means: "Coolant Temp Below Thermostat Regulating Temp." I'd appreciate if someone would let me know what this means and how to fix this. Thank you.

Have you checked/changed your thermostat? It's possible it's stuck open, so the engine won't get to normal operating temp. Just a total guess here.

There is also an outside possibility that the sensor itself is bad. Not common, but not unheard of, either.

I had this code. I replaced the ECT (engine coolant temperature) sensor and the code has not returned. It was only a $4.00 part

I replaced the sensor and so far so good! Thank you very much for all the help. PS much more coolant loss than I expected!!!