S5 NA Weird Coolant light issue
 

So when i start my car and its either cold or lukewarm (doesnt do it on hot starts), often times the coolant light will come on about 5-10 seconds after starting, and stop as soon as i start actually driving. sometimes itll turn on again at the first stoplight i come to, then stop completely until the next time i start the car. This has been happening for about a week.

The morning after it started doing this, i checked my coolant levels. The overflow tank was at its exact full capacity, but there was no coolant in the radiator neck, so i filled that up. After that, the low coolant light didnt come on for a day or two.

Then it started to happen again. I checked the coolant again this morning, and for some reason the overflow tank is an inch or so above the full line and the radiator neck no longer is full (let me know if i was wrong to fill the radiator neck, i thought i was supposed to). I didnt start the car, but put the key to the ON position, and the low coolant light came on again.

Through all this, the car has run perfectly fine and hasnt overheated or anything weird at all.

As youd probably understand, I'd like to know what the eff is happening here. Is this something simple and dumb? A little part i need to replace? A freaking rebuild? I just need to know to calm my nerves hahahaha

if the hoses to the reservoir,or inside it is cracked then the coolant can't backfeed into the rad.
This issue will allow the rad to expel but not fill back up.
So check those hoses.

Another thing to try is to get anew rad cap.

Yep I was gonna suggest that as well. I have seen a bad cap let coolant got to the overflow but since it doesn't hold pressure it won't suck it back into the rad as the car cools as it's supposed to. I've got a Stant right now that works fine so you can pick one up local to try until you can get a Mazda one if you like.

or pressure test the system and cap.

Ordered a stant stock psi. Fingers crossed. The issue seems to happen far less often, but still happens so I dunno. I guess we'll see within a few days

and be sure the seat for the cap is free from debris that won't allow it to seal properly.

Got my stant. Problem solved! Thanks guys.

Okay so the problem was gone for a week or so and then returned even worse. Now it does it until the car reaches full operating temperature, and seems to only stop if I floor it or the car lurches a bit. Sometimes it'll stop for the entire drive, sometimes for only a few seconds.
I still have that stant on there, which does say made in Mexico (I've heard rumors and whatnot) and have yet to try throwing on my previous radiator cap to see what happens.

You may get some ideas of what to check out, so....

I had a similar problem last night on my S5 N/A. After driving for about 400 miles the past 3 days, my Add Coolant light came on as well (this is the only time it's happened since my rebuild). Temp gauge didn't go past the 1/2 way mark, tho' so I figured it was the sensor on the radiator. For some reason, my reservoir tank is always ends up empty, even tho' I fill it and my radiator before I roll out.

I filled up the radiator again, more than usual, but I also checked my radiator's sensor (on top center). Found out the connection was corroded.

Before my rebuild I had a blown coolant seal, and the reservoir would always overflow once it got to operating temperature.

Unfortunately, my current thermostat and radiator cap are NOT OEM, but that'll change soon, as well as installing my hose for the thermowax (part finally came in a while ago from Mazdatrix). Finally putting it back to stock so I can get used to that before I even attempt to mod the motor.

I do remember as I was breaking in the motor, the temp guage did creep up on the freeway (about 6 weeks ago). I caught it in time and shut off the car as I pulled over. Wasn't sure what caused it as my electric fan was working (always on). Hasn't happened again. i have read something about small pockets of air could cause it.

i heard a bad coolant seal could cause it.