Looking for a little advice with my coolant.

I've got a 63 plate audi A3 and the coolant low warning has come up on my dash, I've checked the coolant and it was in the middle of the max and min line, however, when I've opened the tank up its hissed and starts to bubble, rising the coolant up and overflowing it stops after a minute or so. The temperature gauge gradually increases and stops around 90 degrees.

I could top up the coolant and the warning would go, but then appear again after the car warmed up. 

I've checked with the test kits to check if it's a blown gasket, that came back fine, normal black oil for a diesel engine.  After more research online, I was told to check the temperature of the fans when the heating is on, no hot air was coming through,  they suggested the heater matrix. So this has been replaced and the coolant system flushed. 

The warning dissappear for around 3 days, then comes back  on. Opening the coolant tank it still hisses the coolant rises back to where it was 3 days ago.

I've read online it could be the coolant temperature sensor,  ive ordered one online and for the life of me I am unable to locate where it is in my engine bay. Online says its on the passenger side on the return pipe of the radiator, but I can't seem to find it. Could anyone point  me in the right direction or a photo of where it is so I can change it and go from there. 

 

Cheers 

Connor 

Welcome Connor,

Why not suspect the level ‘switch’ or its connection if you know it indicates the level is low - when it isn’t? 

I'd recommend a diagnostic check to see if any fault codes flag up as it sounds like level is fine. 

1 hour ago, Steve Q said:

I'd recommend a diagnostic check to see if any fault codes flag up as it sounds like level is fine. 

I've hooked it up to a carista and nothing came up. I know it's not what a garage would use and a garage one may pick something up. 

1 hour ago, Magnet said:

Welcome Connor,

Why not suspect the level ‘switch’ or its connection if you know it indicates the level is low - when it isn’t? 

Because I have no idea what or where this will be.

You have not mentioned whether it is diesel or petrol fuelled.

56 minutes ago, cliffcoggin said:

You have not mentioned whether it is diesel or petrol fuelled.

Apologies cliff it's a diesel 

Under the reservoir Connor. 

Diesel EGR coolers have a reputation for leaking coolant and blowing back to the reservoir.

On 4/19/2025 at 6:06 PM, Magnet said:

Under the reservoir Connor. 

Cheers Magnet, unfortunately  I took off the coolant tank this morning and couldn't find the sensor. It's either I've ordered the wrong sensor or I'm blind/ looking for the wrong thing. 

This is the sensor I've ordered.