Low coolant level again

I had my intake manifold gasket replaced last year. But recently I got the following problem: "Low coolant level" is on when the engine is cold, for example, when first starting the car. After the driving for about 10mins, it just went off and never comes back even with >3 hours driving. So, do you have any clue what the problem is? If it is the gasket, I think it will come every some time. So this time it is different.

I checked the coolant level in the plastic pump, it is normal.

By the way, previously my temp guage is always kept at the same level. After the t-stat changed last year (along with the gasket), it will oscillate. But when it reaches around 210F, the fan will work and bring it down. Is this normal?

Car: 2002 Impala, 3.4 V6 SFI, 107,000miles

For your coolant light worries- Does the light come on on startup? The car does a self check briefly when you turn the key to on, oil light, check engine light etc illuminate. If the light comes on at self check and goes off within 3-5 seconds, you may have had a sensor error. (Considering you say the coolant is not actually low.) If the light does not come on you may have wiring or other issues.

And yes, tstat op is normal

The light comes on at startup. Maybe it's a sensor problem.

And is it possible the replaced gasket has a problem again?

For your coolant light worries- Does the light come on on startup? The car does a self check briefly when you turn the key to on, oil light, check engine light etc illuminate. If the light comes on at self check and goes off within 3-5 seconds, you may have had a sensor error. (Considering you say the coolant is not actually low.) If the light does not come on you may have wiring or other issues.

And yes, tstat op is normal

If your coolant level is fine, then the gaskets are fine. What may have happened is sludge or oil residue from the faulty LIM gaskets may have gotten into the coolant level sensor, causing it to trip the light. The other possibility is that there is still air in the cooling system from the t-stat change.

The coolant level sensor sits pretty high in the radiator -- when the coolant is cold, it may drop below the sensor until it has warmed up -- one thing you can do is "top off" the radiator when dead cold, when the level in the radiator will be the lowest, and make a viaual note of the level in the overflow tank -- after a few drives, check both again when cold -- one issue may be that the design of the overflow tank (or air in the system) may not allow the coolant in the overflow to return to the cooling system, so you'll have a constant low to high reading in the radiator. I would consider this somewhat normal, and not necessarily a problem that you can do much about.

Basically, all this may be normal, and the best the system can do -- ambient temperature is everything, and small changes can make the very sensitive PCM confused. It is Spring -- temperature changes can exceed 40 degrees from one day (engine startup) to the next -- the GM PCM does not compensate for these kind of rapid changes very well, and instead likes to adapt slowly to extreme variances.