Recently, I am getting a low coolant warning. I put more coolant into the reserve plastic coolant holder and the message went away. But now it came back?

Start it and look at it while its running. Also try giving it gas to raise rpm's. You should see it then.

If you are talking about the low coolant warning light, be sure and check the radiator along with the overflow tank.
If the cooling system is full, the sensor has failed.

Did you over heat warning message come on " Urgent shut off engine" or check coolant message? My coolant message came on but once car warmed up, it was late summer. No leakage but coolant level was low; added some coolant. Then it happened again but once car warmed up the check message disappeared. Then a few days later I got the Urgent Shut off Engine message! Thankfully I was able to pull off road and shut down immediately. Had it towed to Caddy shop. My water pump had gone out. New Pump and replaced all hoses. Then a year later check coolant light came on again, once car warmed up, light went off. This time tho a small leak in the radiator. My husbank put in a new radiator. Car has 117K miles and a few weeks ago outside temp 35, running late for work and didn't allow car to warm up but just a minute or two and when I puled away the check coolant light came on; but once the car warmed up (temp gauge needle in center) the light went out. Being the car is 17 years old I try to treat her like a baby and not push her so I do let the engine warm up before driving away. Do you have to add oil between oil changes? This is normal for the Northstar engine; your regular garages or full service stations don't know this; also make sure if you aren't using an authorized Cadillac garage that they put the right amount of oil; they want to put less than what is required. Not sure if this is really a solution more comments than anything... :)

the problem is in most cases in the probe for low colant light try to clean it if that dont work replace it. good luck. Hans

if rad full then its the sensor in the header tank

Check the coolant top up reservoir. Low coolant sensor is usually in the overflow/reserve container.
Wash out and clean any dirt or corrosion from the sensor prong.
Refill to correct level with fresh coolant, then recheck alarm operation.

It sounds to me like you're low on coolant. When you checked the coolant level, did you actually open the radiator cap and make sure the radiator is full? The plastic tank you generally add coolant too is actually an 'overflow' or 'reserve' tank. There is a hose that runs from that tank to the top of the radiator so if the radiator gets low on coolant, more coolant -should- run in from the overflow tank. However in practice this does not always work. I have a Jeep that leaks coolant and experiences this exact same problem. Coolant will not run from the reserve tank into the radiator because the hose is plugged up, and whenever the radiator gets low on coolant the temp gauge will go real high and then drop, and the heater doesn't work well. This is because the cooling system of your car is supposed to be a closed system, full of coolant and no air. When coolant leaks out, the space it used to occupy is now occupied by air, which does not transfer heat well. When 'air' is passing through your cooling system, no heat can be transferred from your engine to the heater and radiator, resulting in a hot engine and no heat at the heater. Then when a pocket of water passes through the system, the temperature gauge quickly falls as the water absorbs the heat from the engine. The hot water that cools the engine is where the heater gets it's heat from as well, so when water passes through the heater core, the heater works, but when it's filled with air, it doesn't.

you might fill it at the radiator itself..the overflow dosent allow all the water to get in where you need it to sometimes.i would check the radiator itself

1 IT IS OVER HEATING 2. THE RESERVE IS LOW NO COOLANT OR A DEFECTIVE SENSOR I THE RESERVE TANK

I have a 2005 Ion that had been displaying the "coolant" message for quite some time. The car runs perfectly, never overheats and the coolant bottle is always full to the "cold fill level" line when cold and slightly higher than that when hot due to the expansion of the fluid. (all completely normal) I had a strong feeling that the coolant level sensor in the coolant bottle was bad and had read about how it needed to be replaced by a tech and all but I wonderred if there was a way to defeat the sensor since I check my levels all the time anyway because the sensor was really just giving false info anyway. I couldn't find a tread involving a Saturn that talked about a solution other than having a tech charge me lots of money to replace the whole bottle. I found a way to bypass the sensor for a BMW though and thought I would try it. It worked perfectly. I plan on replacing the bottle eventually but right now money is tight and I don't need the sensor to tell me what my own eyes and my car's temperature gauge can tell me for now. Here's how you do it.

1. Unplug the sensor harness from the bottom of the coolant bottle.

2. Find a large paper clip and use needle nose plyers to make a "staple" shaped jumper that is about 1/4 inch wide (about the distance between the two contacts on the harness) and about 3/8 inch long.

3. push the jumper you just made into the end of the harness connector.

4. Start your car and ensure that the warning is no longer displaying. You can also insert the jumper while the car is running (be careful, the coolant bottle will be hot) because the harness is removed from the sensor the coolant warning should already be displayed. When you insert the jumper the message should go away imediately. The coolant level sensor has only one job and that is to tell the computer when coolant is low so you don't have to worry that you may be causing any other problems by doing this. If the coolant light doesn't go away you may have a break in the wire somwhere or a bad connection to the jumper.

5. If all works well, use electrical tape to seal up the end of the harness so moisture doesn't get in. Also it's a good idea to secure the harness to something nearby so that it doesn't flop around when you are driving.

All done. Sorry this was long but I hope that it helps someone. That warning really has been irritating me.

Keep in mind that this method does disable a key safety device that is designed (poorly) to warn you in the even of a coolant leak. Keep an eye on your temperature gauge and check your coolant level regularly. Get the bottle replaced when you can.