2000 Kia Sephia check engine light cold weather

My 2000 Kia Sephia has 105,000 miles. This winter my check engine light came on and the engine seemed to idle rough. The interesting thing was that after about 24-36 hours of a warm spell of outside temperatures above 50F, the check engine light would shut off. Then it would light up again after the same period of time with temperatures back in the 30's. Now that it is spring, the check engine light is off, but the car still seems to idle rough. Any ideas? Thanks
All maintenance and tune items need to be up to date and get a code reading. Don't wait too long as anytime an engine isn't running its best it's hard on o2 sensors and converters,
We've had a cold spell with outdoor temps in the right range for my check engine light to go on again so I immediately got it scanned. The code is P0128, coolant temperature sensor. Do you have any further advice now that I have the code. Of course I will get this sensor replaced. Thanks for your time and advice.
If you've got the code for the CTS it's worth doing it IMO. Double check wiring and connector look good and it should be located near thermostat and plug look like this so you know that's the one......

Duralast / Coolant Temperature Sensor Part Number: SU5272 Price: $34.99
Warranty:
2 YR
That just threads into coolant and tells the PCM how much fuel to deliver. You can check this one with an ohm meter, put it on warm water then in freezer and read ohms or take the chance and toss it. If high enough up on the engine when cool/cold it just might not leak much to removed it without draining the whole system but be ready for drips anyway. Good luck - I think you've found it, T