Red coolant warning light


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Aug 25, 2012
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65 plate S3.

No real problems so far.

Just did about 30 minutes motorway driving pretty fast and then stopped. A few minutes after heading off I got the red coolant warning saying to turn off engine and check coolant. I pulled onto the hard shoulder and had a look. I got a brief release of steam and then the coolant water bubbled out the reservoir.

Having seen there was coolant there I went on and the light came back on after a couple more minutes. I stopped again but was only a few miles from home by then so headed off again. This time the light didn't come back on (was just city driving).

Any thoughts? Bad sensor? Should cooland bubble out of the reservoir?

One other thing, probably not really relevant, I turned on all the heaters to max to try and pull as much heat from the engine as possible but the air wasn't very warm coming out.
Not sure on S3 in particular but engine coolant is usually pushed through a heater core for fans to extract hot air from when the heating is on, so if you've had a cooling light come on and you're getting poor heating that definitely points to a problem.

Coolant is usually about 90°C when running and bubbling isn't a good sign. Means air is getting into the system somewhere and often a head gasket.

Worth getting a pro opinion on this I think.
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Multiple things here:

How long after your high speed run before you turned the engine off? If it was straight away eg going into a Services then your exhaust manifold & turbo would still have a lot of heat in them and could boil the coolant even with the run-on pump.

Cabin heater: could be blocked heater matrix or stuck valve for the cabin heater. Check your coolant reservoir - is there a bag of silica beads in there and has it burst?

After letting the car cool down, where does the coolant level sit in the reservoir? Does it change if you open the expansion cap?

Could also be a stuck thermostat in which case that will be leaking soon if not already.
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That ks both, much appreciated.

It was straight off the motorway when I had to open the cap so still very hot.

Can't see any beads in the tank. I topped it up yesterday once cold and then checked again this morning and wasn't any different. There didn't seem to be any change in pressure when I checked this morning but will check again later after driving.

I'm OK only doing short journeys this week and will keep an eye, and I've got it booked in for Friday with a local audi specialist (not audi dealer, they could look for almost 3 months ). Hopefully they'll find the issue.

Thanks for the advice.
So the car went in to be checked and they couldn't find anything...
They said the coolant looked black which they said was weird. They did say it wasn't oil though so I guess maybe a hose is perishing or something.
They did a full flush and refilled the coolant.
All was fine yesterday but then today the light pinged again when on the motorway. I switched off the engine and restated and the light stayed off.
So who knows!
So the coolant level is above the minimum (but below the maximum) on the reservoir when the car is cold? Assuming so then perhaps it is a failing level sensor (or wiring to it). Perhaps you should try to monitor the coolant temperature to make sure its staying in range (you can do this with a OBD dongle and app like torque pro).
Sorry Damo, didn't spot your reply.
Have only been town driving for a while so not had any problems but just did a long journey and had the sensor go off again.
It was fine most of the way (2 hours of motorway about 70mph) but then went off when I went fast.
Stopped the engine but was too hot to check the level. Started the engine again to see if it cleared and it had so I carried on again and was fine.
So I did some testing... Seems I can trigger the alarm by going v fast (like 95mph), even briefly. Normal motorway speed is fine.
Seems odd. I'll book in with audi but they won't be able to check for a couple months.
I had a similar problem with mine at the beginning of the year. About an hour of motorway driving, the low coolant level light came on. Pulled over and turned engine off and checked coolant: it's the max line like always. Restarted, no warning. About a week later it came back - still nothing wrong with coolant level. For a couple weeks it was intemittent, sometimes on, sometimes off. I spoke to VW to enquire about a new sensor, they said it comes already fitted to the header tank, but before I go ahead with a replacement I should pull the plug off the sensor and check the terminals make sure there's no corrosion or anything. I did that, all looked good, refitted the plug - hasn't done it since. And that was over 3 months ago.

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