[2004 Zx3] P0128 Code - Temp Gauge works fine.

[Vilita]

2004 Ford Focus Zx3 with 145,000 miles
4 cyl 2.3L Automatic Front Wheel Drive

Got P0128 code ( Coolant Temp below Thermostat regulating temp) on 2004 Ford Focus ZX3 with 2.3L PZEV Engine. No noticeable change in performance, temperature gauge on dash still functions normally with temperature reading nominal. Where should I start, to figure out the cause of this?

Temperature has been cold recently outside, heavy snow and cold conditions while the car did not drive for a week before the light came on