If your engine doesn't warm up fully, your ECU won't go into closed loop (so you'll get bad gas mileage). A few degrees is okay, but if your engine doesn't get up to operating temperatures, even things like its geometry can go out... because things like tolerances for thermal expansion of metals and viscosity requirements for oil clearances won't be correct.

Just change that $6 thermostat. 2 bolts hold it in.