Recently I've been experiencing an intermittent fault with the cooling system on my car (a 1.2 TSI Fabia) where the red coolant light starts flashing and a warning buzzer sounds when I'm driving.  When I pull over and restart the engine the warning light goes out immediately and the car always has plenty of coolant and doesn't show any other visible issues or signs of overheating.  I've been led to believe that the flashing nature of this light means that this is likely a sensor fault or an incorrect low coolant warning rather than an overheating issue (but maybe someone could confirm?).

 

Notably I never have any issue on short journeys (<15 minutes) and almost always just about 15 - 30 minutes in, normally shortly after joining a motorway or similar - so realistically just when the engine would be approaching its full operating temperature.  This suggests to me that it may be temperature related - if not overheating then something triggered by the engine warming up.  Generally once restarted the fault won't reoccur for the rest of the journey and I can sometimes go several long journeys (200 miles +) before I experience the issue again.

 

I have been to a garage previously and they did replace a coolant sensor that they suspected of being at issue (they unfortunately couldn't reproduce the fault in the garage, only review the sensor data).  This did appear to fix the issue for several months but it has now returned (possibly a new fault or just chance that it worked again for the last few months).

 

Does anyone have any suggestions as to what the most likely cause is and if there's any straight forward way of telling what fault caused the warning light (overheating, temperature sensor fault, coolant level sensor etc.)?  My car doesn't have a temperature gauge unfortunately but I am planning on getting an ECU reader so I can at least see what values are being reported when the warning light is triggered.

 

Thanks in advance for any help you can offer.

8 hours ago, SkodaMark2 said:

an intermittent fault with the cooling system on my car (a 1.2 TSI Fabia) where the red coolant light starts flashing and a warning buzzer sounds when I'm driving

This is likely a very common fault with the level sensor in the expansion bottle. On a cold engine remove the cap, and look into the bottle. If you can see a couple of metal prongs sticking down into the coolant, rub a flat blade screwdriver down each of them a few times. If you can't access the sensor, you need to replace the expansion bottle, which cost ~£20 the last time I looked.

Thanks for the advice, the metal prongs don't seem to be accessible in the expansion bottle so I'll look at getting it replaced.

Just so you know, carports4less charge just £6.35 for an expansion bottle to fit a Fabia and many other VW branded vehicles...This IS with the discount code applied.  Eurocarparts are around £8, plus but might be easier to source a branch.

Just as a quick update for those interested - I attached an ODB2 reader to my car to check what was triggering the coolant light each time and it turned out the car was actually getting very hot (~130 degC), so not just the bottle sensor on this occasion.  This was diagnosed as the thermostat sticking so not allowing the coolant to circulate - I've now had this swapped out and the car seems to be running much better.  Thanks everyone for their help too.

It's good that your engine survived that.  No thanks to this design decision:

On 11/09/2021 at 01:33, SkodaMark2 said:

car doesn't have a temperature gauge

 

7 hours ago, Wino said:

It's good that your engine survived that.  No thanks to this design decision:

 

 

Ain't that the truth!!!!....................cheap skate manufacturers !!!

...Not really sure thats fair!  The light and sound was warning you there was a fault, you thought it to be the sensor, it was not and then found it to be the thermostat, which on an older vehicle, pre-dating so many sensors, etc, would have been the first port of call or at least a close second after checking the fluid level and testing for air-locks.

 

So the car told you it had a fault but as often the fault lays with the coolant sensor in the expansion tank, the thermostat got overlooked and only found later. Not really a design issue of the car.

A coolant gauge needle bouncing off the top end-stop would've been less ambiguous.

It wouldn't have cleared/reset with cycling the ignition either. 

 

Edited 20 September, 20214 yr by Wino

1 hour ago, mrgf said:

So the car told you it had a fault but as often the fault lays with the coolant sensor in the expansion tank

Much more often the fault is with the level sensor, as I described. You can actually get this "overheating" fault on the first cold start of the day, so you're certainly not going to presume the engine is actually overheating.

How do these coolant level sensors work? Do they pass current between the two prongs but in the absence of any coolant the warning is triggered? If there is sufficient coolant the circuit completes? 

1 hour ago, Moribund said:

How do these coolant level sensors work? Do they pass current between the two prongs but in the absence of any coolant the warning is triggered? If there is sufficient coolant the circuit completes? 

Yes, that is correct. There was another posting a while ago, where someone spotted that the design of the level probes had been changed - possibly in response to issues with previous type. 

29 minutes ago, Warrior193 said:

Yes, that is correct. There was another posting a while ago, where someone spotted that the design of the level probes had been changed - possibly in response to issues with previous type. 

Thanks for the quick response Warrior. I’ve got an ongoing problem with the coolant warning being triggered (from cold start) but the level is above minimum and it doesn’t seem to be losing coolant. I’ve noticed that once the warning goes off a temperature sign appears but it goes off intermittently. This coolant tank was only fitted two weeks ago when the heater core was replaced. I’ve tried cleaning the prongs this morning and will see if it makes a difference otherwise it’s back in the garage.