2003 Galant with engine code P0128 going on and then back off after a while

[mkohanek]

I have driven a 2003 Mitsubishi Galant 4 cylinder for 13 years (starting in December 2002, when I bought it new).

For about 6 months now, I have had my check engine light come on, and then about a week later it will go off. Then in another week or so, back on, and repeat.
When I have had the code read, it was P0128, which from what I have read means:

Coolant temp is always low
The powertrain control module verifies that the engine is at a proper operating temperature

Check coolant level
Thermostat defective
ECT (engine coolant temperature) sensor defective

I am hoping to narrow down what it might be, and get tips on how to go forward. The thing that might help in doing that is the fact that the engine light seems to go on and off, so the problem is intermittent.

One thing I seem to notice, is that as the gas tank gets closer to empty, that is when the engine light seems to come back on. Then a little after I fill it up again, the engine light seems to go off.
I cannot say for sure if this is a correlation, or even if that is always the case, but it sure seems to happen an awful lot in that manner.

Does that just mean a faulty sensor or thermostat, or could it be something else? Does this information help narrow down the possible causes at all?
Any tips on the best away to proceed? I do not mind doing the labor myself, as long as it does not require anything too drastic.

Thanks

[Crvett69]

is your car temp gauge in the dash reading about in the middle of the gauge? if its reading low most likely your thermostat is bad and needs to be replaced

[mkohanek]

Originally Posted by Crvett69

is your car temp gauge in the dash reading about in the middle of the gauge? if its reading low most likely your thermostat is bad and needs to be replaced

I have been keeping an eye on it, and it seems to stay right in the middle