Odd coolant light problem, among others.
 

About two weeks ago, the plastic thermostat housing on my S5 N/A developed a hairline crack and some coolant started spraying out. I ordered a new housing + thermostat on mazdatrix. In order to keep the problem from getting worse in the meantime, I cut up an old hose and clamped it around the housing + dumped in half a bottle of bar's radiator stop leak (which made me pretty nervous).

Every now and then, my add coolant buzzer would go off; understandable, considering there was a crack in my housing and some air was probably getting in. I drove the car very gingerly until my new parts arrived.

I got my package and installed the new thermostat + housing. Then I opened the pressure cap and let the car idle for 10 or so minutes with the heater blowing.

Oddly, the "add coolant" buzzer still persisted -- especially when at a dead stop. As soon as I took off (or in some cases, reved the engine from a stand still) the light would go off. Sometimes it came on while at low RPM while I drove, but mostly it only came on when stopped.

Frustrated, I went out and bought a new upper radiator hose, desperately thinking that the old one wasn't sealing to the new thermostat properly, allowing some air in. But I mean, it's clamped on there pretty tight, so that shouldn't be an issue.

I haven't put the new hose on because the warning light stopped coming on for the most part. (It used to come on at every stop. Now it just comes on every other drive, I'd say, to sort of remind me that the problem hasn't subsided.)

So..... what the hell could this be? Any ideas? The car isn't leaking any coolant, it never smokes, engine is in great condition, etc. Oh, and the sensor is still good. I really want to emphasize the point that the buzzer comes on at low RPMs/stops.

Oh yeah, when my thermostat housing cracked I went from getting around 18 city MPG to about 12. Yeah. Considering gas prices, I'm not too happy. Could these problems be related?

Thanks a lot guys.

Weird. I just had the same exact problem last week, down to the hairline fracture on the housing and new parts from mazdatrix. But after purging my coolant a couple times the problem went away. Their is a thread somewhere here that has a good coolant air purging procedure.

You have bubbles in the coolant, fill through the top fill opening and bleed the system through the bleed caps.

i have the same problem bro. That light glares at me. It huants my dreams. I have checked everything...like you. Except the radiator stop leak which I'm gonna buy tomoorow now that it has come to mind. Sorry I can't help. Micah

The stop leak has coated you sensor reducing its ability to conduct (filming). Remove the sensor and clean it with dish washing detergent, drop your coolant refill with water run for ten minutes and drop it again run it without the cap and don't rev it. Drop it again refill and add inhibitor and your clean sensor and radiator cap bleed your system and it will be good!
Michael Smith

So basically just flush the system? Thats wierd, my rx7 is having this exact problem....

How to correctly fill the cooling system and bleed it of air:

1. Put the heater in the HOT position (you'll need to turn the ignition on first to operate the stepper motor). Leave the heater on until your test drive at the end is finished.
2. Make sure both the engine and radiator drain plugs are installed.
3. Remove the filler cap (on top of thermostat housing), radiator cap and (on S4's only) the bleeder on the side of the top radiator neck.
4. Begin filling the water/coolant mix through the radiator neck until it reaches the top, then replace the radiator cap.
5. For S4's, continue filling through the filler neck until the bleeder overflows, and replace the bleeder cap. Don't overtighten it or you'll ruin the plastic plug.
6. Continue filling though the filler neck until the water/coolant reaches the top.
7. Start the engine and let it idle. The water level will probably drop a bit when the engine starts as the pump pushes the water into air pockets. Keep topping up the system as necessary.
8. Keep the engine running until the thermostat opens. You can tell when this happens as there might be another drop in water level, and the top radiator hose will start to warm up.
9. When no more air bubbles are appearing in the filler neck, replace the filler cap.
10. Fill up the overflow bottle to the correct level.
11. Go for a test drive, keeping an eye on the temp gauge. Make sure the engine reaches full operating temp but does not exceed it, and check the heater operates properly.
12. On your return, once the engine has cooled down, recheck the level at the filler cap (not the radiator cap) and the overflow bottle.

:)