As Tim suggests, clearing the code doesn't solve the problem if it's not a random positive.
The engine temperature gauge is fine
Based on the information that you've provided so far (and using Tim's link as a reference), it seems likely that you have either a bad engine coolant temperature sensor. The implication from your PS is that the car is, in fact, reaching operating temperature just fine (so the thermostat must be working properly).
If you had said that the car was fine at highway speeds but overheating in city driving (or other abnormal temperature ranges), I'd be inclined to suggest that the thermostat is the problem.
All that said, it is worth noting the caveat on RepairPal:
An engine coolant temperature sensor code is not always caused by a
defective sensor. It can be triggered by a defective thermostat,
defective wiring, rusty coolant, or an overheating engine.
So, after all of the above, it's not impossible that your coolant is just full of goop.