Q: how to get to thermostat or sensor that causes trouble code po128 on 2002 Saturn SC2

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engine has never over heated but the check engine light came on and stays on the trouble code is po128 and last night the red light came on that looks like water level on radiator but it is full.
(1) Answer
1. Drain coolant from the radiator (1) and engine drain plug (2) located at the right front of engine.
2. Disconnect lower radiator hose at thermostat housing. Use Snap-on Tool HCP10 or 20 (or equivalent).
3. Remove two bolts in water inlet housing.
4. Remove water inlet housing and thermostat assembly.
5. Remove O-ring.
* The O-ring gasket must be replaced whenever the thermostat housing is removed.
* The thermostat element should not be removed if the assembly requires testing.
6. Remove the thermostat element using the service tool provided with the replacement element.
NOTICE: The thermostat will not function correctly once it is contacted by oil. If oil is found in the cooling system, it must be flushed and the thermostat's cartridge replaced.
7. Inspect the thermostat components for damage, seat deterioration, etc.

I would start with the thermostat and see what happens. make sure you use new coolant.