Trouble Code P0128 on your Saturn L300 does indicate Coolant Temperature Below the ECM's expected value however this does not indicate the Engine Coolant Temperature (ECT) sensor is defective. It simply means the engine coolant isn't getting hot enough.
We first recommend you ensure your Saturn's radiator has the recommended level of coolant, and than suspect a defective thermostat.
Once replacing the thermostat, clear the trouble code, drive your Saturn L300 for 200-300 miles (in order to reset the emission monitors), and once ensuring the check engine light has not returned you'll be ready for a smog check retest.
