Cooling system warning
 

So I topped the coolant up yesterday but only with water? Same as I have done with pretty much every car I've had. But upon starting the a8 this morning the coolant system warning light comes on.

Is this because I have used water or just complete coincidence?

TIA

The cooling system is sealed, if you are having to add water then it has a leak.

Adding just water is not recommended as you are constantly diluting the anti-freeze solution, you should add a pre mixed to the correct ratio anti-freeze and water mix. The anti-freeze must be the correct type type, not sure on what is used on the D2, but if you mix the traditional blue glycol based anti-freeze into a car which uses G12, G13 or one one of its derivatives then the chemical reaction will sludge the coolant.

The warning light coming on is quiet common on the VW group cars, the sensor in the header tank gets gunked up and triggers false alarms. Sometimes just turning the engine off and on again is enough to clear the fault. Ultimately replacement of the header tank is the only way forward.

To add to Manny's already thorough answer - the reservoirs can develop a crack and therefore a leak around the outlet pipe at the bottom edge where the coolant take off pipe goes off towards the engine - then that lets enough out for the warning to trigger as well

Not gonna lie was only just below the max line and I though, why not just top it up.

Wish I'd left it now!

Will check for leaks but seems more than coincidence since I've touched it it's now got a problem!

I guess I should use g12 coolant?

Thanks, will get her home and check it out

Was it hot or cold at the time?

All sorted!
Thank you all for the help

Just getting back to where Paul was heading with this - the 'Max' mark on the reservoir is the hot level. The 'Min' mark (just under the lip of the reservoir top) is the cold level. When cold, if it's at the lower mark it doesn't require filling.

Correct coolant is G12, which has been superceded by G12+ (or ++). All are compatible with each other.
Don't mix G13 (new standard) with G12.