I have a 2003 Impala with about 137,000 miles on it, I bought it back in Dec of 2014. Over the almost 3 years I have had it I have had zero issues with my temp of the engine, however I have never changed out the antifreeze and have no clue the last time it was done. I check it from time to time and have had to add some over the 3 years (less then a full gallon) and the color in the radiator is an off greenish color. With winter coming up I am looking at having the antifreeze drained and replaced and have all the hoses checked. Is this a good plan? Also should I have the car thermostat replaced since I don’t know how old it is even though I have had no overheating issues?
Yes. Have the antifreeze replaced, and have the cooling system pressure tested for leaked. I don’t think you should have had to add a gallon of antifreeze in this time period. A new thermostat would be a good move.
YES!!!
This one is debatable. And I guarantee there’ll be debate.
I don’t believe in changing out a good T-stat just for GPs. Others here disagree. IMHO it’s your choice. I drive my cars for hundreds of thousands of miles without incident… but I DO change out my coolant periodically. That, I believe, is the key.
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My experience with thermostats has.been that they stick open when they fail and you don’t have any heat. The engine doesn’t get up to operating temperature and you waste gas. The last.time I remember a thermostat stuck in the closed position was on my Dad’s 1939 Chevrolet. Whether thermostats have improved on modern cars I have no idea.
Na no problem with heat, car warms up just fine and I get a nice amount of heat to warm the inside of the car when needed.
I have one of those Impalas. These are some of the best cars ever made, especially the 3.8L engine models!
Mine has a 3.8L engine and I have changed the coolant (and thermostat) on it. I can offer some DIY advice, but it sounds like you’re going to pay to have it done.
That car came filled with Dex-Cool coolant. It’s (red/pink). That green stuff was put in by somebody. It’s subject of great debate and that green/yellow coolant will work, however I’ve run Dex-Cool for over 300,000 miles. DO NOT mix them (green and red).
These systems vent the coolant reservoir to outside air and some loss to evaporation is normal. I’m not sure how much “less then a full gallon” actually is. A gallon in 3 years sounds a little excessive, but half a gallon not so much, though.
You need to change the coolant. If it’s thoroughly flushed out you could use Dex-Cool, but you probably will be Okay to use the Yellow universal coolant in this one. I’m not sure the green stuff is even available any more.
Do you have the 3.8L (3800) or 3.4L (3400) engine?
CSA
Consider changing the automatic transmission fluid, too. Have the transmission pan removed, a new filter installed, and fluid replaced. I would NOT “flush” the transmission.
The fluid to use is Dexron-VI (six). Do Not use a more universal fluid. I would not use Dex/Merc, etcetera. Any brand of Dexron-VI (GM licensed specification) will be good.
I believe this car came with Dexron-III (three), but it has been superseded to the VI, a synthetic, compatible fluid.
CSA
Transmission fluid was just changed out before I bought the car in 2014 and it’s still a nice red color with no bad smells. As for engine I believe it’s the 3.8L.
That’s it! That’s what you want! 
CSA
I would not change the thermostat. For several reasons:
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not broke, don’t fix
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There is a significant chance you will get a defective one as a replacement, or it will be installed incorrectly.
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How many miles since the trans fluid was changed? I changed it every 30,000 to 40,000 miles on my last car. When I sold the car at 190,000 miles, the engine and trans performed very well. The used car manager even commented about how well it ran for the miles.
right now sitting at about 33,000 miles since it was changed out.
It’s easier and cheaper to do everything at one time, so after 5 years I had the Dexcool replaced, the thermostat, and all of the hoses. So yeah at least change the fluid and thermostat.
If the coolant is green it has been changed from Dexcool once in it’s life, I know there are dexcool lovers on this site, but sticking with a green, global antifreeze is my recommendation. Sounds like the previous car owner may have been following the manual, my guess would be the coolant was done per mileage in the manual.
I’m not a “Dex-Cool lover,” but rather an individual who believes in using manufacturers’ specified fluids and filters in my cars.
I currently own 5 GM cars that specify Dex-Cool coolant and that’s what I use in them. I have had other GM cars that also specified and used that coolant. In many hundreds of thousands of miles I’ve never experienced any problems using it.
It stays nice and clean and will truly go for 5 years without issue.
Why would I experiment with something not specified? Why recommend that somebody else not follow manufacturers’ recommendations and experiment when they need advice?
I use AC Delco filters and spark plugs (iridium 100,000 mile), as specified. Again, never a problem. I will only use the specified Dexron-VI transmission fluid as specified.
I would never try and out-smart or out-guess the folks who designed, built and warrant these vehicles. Go by the book and long, reliable service will result. 
CSA
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I gave my recommendation on this above. I agree that once it has been improperly changed to green coolant that it either needs to stay wrong or thoroughly flushed and refilled with the proper coolant. Again, don’t mix red and green coolant!
CSA
I’m not a Dexcool lover either and immediately wanted to pull the Dex out. But after talking to two GM dealers and a couple independents, I just stuck with it, and it has not been a problem. I do believe though you have to be absolutely careful to not introduce air into the system and keep an eye on it to make sure it is not gunking up.
I think improperly changed and stay wrong is, well to put it in simple words wrong.The green global coolants work fine and the advantage for older cars is if there is air introduced into the coolant system you will not be a victim of dexcool sludge.
I try to change coolant every three years. Many recommendations I see are for two year coolant changes, so yes, your idea to replace the coolant before winter sets in is a good one. The coolant reacts with the metals in the engine and tends to becomes acidic over time, and acidic coolant can damag stuff inside the engine if you delay. If it were my 2003 car and unsure of thermostat status I’d replace it at the same time as the coolant. I’d first put the new one in a pot of hot water on the stove with a thermometer and make sure it opens and closes at the correct temperatures and dimensions. A few other tidbits about thermostats, when you remove the old thermostat, pay careful att’n to its orientation, so you can install the new one the same way. All of them have an in vs out orientation, and some of them have a top and bottom too, often identified by a little hole. Make sure the new thermostat looks exactly like the old one. Thermostats don’t usually come with the gasket, as the gasket is make/model/year dependent, so make sure you ask for the correct gasket too. Make sure you understand the air bleed procedure for the cooling system of your vehicle, otherwise you may experience overheating after replacing the coolant.
Come on guys, you’re perpetuating a myth! Don’t believe everything you read. You can relax! Don’t worry about introducing air into the cooling system. 
That’s not a problem. I’ve done everything one can do with Dex-Cool.
I believe some very early original Dex-Cool had some problems, but was reformulated years and years ago.
About the only ways I know of to screw up Dex-Cool is to never change it or to mix green coolant with it. 
Reading the instructions on the container will inform the user.
Add only Dex-Cool to Dex-Cool and everything is copasetic. In other words, read the Owner’s Manual that came with the car. GM specifies it!
I use Dex-Cool and always have. I sleep very well at night, too! 
Now, that doesn’t surprise me! 
CSA