Hi. I have an intermittent issue with the coolant warning light coming on. This is on my 2017 MkIII Facelift vRS TSI hatch. Just over a year ago I had a coolant leak, which resulted in the replacement of the temperature housing. Since then it's been working fine until just after Christmas this year when on a long journey (Bristol to Swindon, the return leg) the coolant warning light came on towards the end of the journey, just before turning off the M4.

Since then I've had the warning light come on a number of times, all in very different circumstances, including one time when it came on after only driving for about 5 minutes within town (hadn't driven it that day and the weather was cold). The other times have mostly been when driving for 45 minutes or more, e.g. driving to Gloucester from Swindon (happened on the outbound journey) and didn't happen on the way back - I pulled over, turned off the ignition, checked the coolant level, which was sitting at the upper line and hadn't gone down - started the car again and it was fine the remainder of the journey.

I had a service and asked them to look at the coolant, they did a pressure test and all was fine and of course the warning didn't show for them.

A few days later I did the same journey again (Swindon to Gloucester), this time it only happened on the way back, tried stopping for a few minutes with the ignition off but it came straight back on again after about a minute of driving. Stopped once more but it came back on again when I started driving. The temp gauge didn't move beyond normal running temp of 90.

I figure I'll take it back to Skoda who might just do the usual checks again, charge me for the pleasure and send me on my way. I am tempted now to just start asking them to replace bits until it gets fixed. So what do people recommend? As it seems to be intermittent, I wonder if it's a dodgy sensor somewhere, but this is just a guess.

10 minutes ago, djkarlos said:

So what do people recommend? As it seems to be intermittent, I wonder if it's a dodgy sensor somewhere, but this is just a guess.

Tell an independent VAG specialist this story. They may have a suggestion (my thoughts being around the expansion bottle).

Try getting the fault codes read first.

Assuming that this warning light query refers to the coolant level, not overtemperature, as temperature gauge is reported as staying 'at 90' - I'd suggest this intermittent fault points towards faulty coolant level sensor probes in the expansion tank.

First check sensor wiring connection, then try gently scraping the probes inside the EV tank with a screwdriver.

This fault has been reported previously by others. Suggest simple and cheap replacement of EV tank will probably fix, can even be DIY.

Definitely sounds like tank probes. You can clean them, by either scraping or wiping clean, but need to have something to reach in tank. Easier if you remove tank and drain and then refill exp tank.

There's only two pinch clamps to remove. Hoses will be hard to get off first time though.

One word of warning, mine did this after mixing oil with coolant from the oil cooler. If there's brown gunge on the probes that's what your coolant problem is but the fix is a fair bit of work, probably a little more or at least same as the the thermostat housing.

Assuming that this warning light query refers to the coolant level, not overtemperature, as temperature gauge is reported as staying 'at 90'

The exact message that displayed on the dashboard was "Check coolant! Owner's manual!" so I assumed it was related to the coolant level....

I'll try giving the sensor prongs a clean and see if that helps.

The other issue that is prevelant in earlier MK3's - is wicking of coolant up the sensors and into the sensor wiring. Loads of MK7 Golf's also suffer this problem. The symptom you report; intermittent issues with sensor readings even though the coolant is at the right level - is an issue with the coolant breaking down the electrical connection into wiring loom. Super bad news, but you can check if this is an issue for you by following the sensor wires back to where the wires join the rest of the loom and squeezing with your fingertips back to the connector. If you see coolant coming out the end of the connector (which goes into the sensors in the Coolant Header tank), then you get an car electrical specialist to trace back the wires to where there is no coolant wicking/hydroscopic action and cut/splice new wires. It seems that for some reason, the coolant is getting past the packing in the electrical connector on the coolant tank. I think its more a problem with the header tank - and if this is an issue, you also need a new header tank to stop that wicking action from getting into the wiring loom again.

Edited 24 March, 20251 yr by varaderoguy

That is the lower coolant level warning although, there maybe other circumstances where it also indicates something else to do with cooling system..

My moneys on the prongs, but as mentioned, it can be a sign of something else. Coolant migration can be a a nasty problem. But less common. I caught before it happened on mine by taking the connector off the plastic tank when engine was hot and I could see a slight bubbling when sprayjng a soapy solution on the connector recess onn the tank..

The terminals where they come through the tank tend to be the weak point!!

Edited 25 March, 20251 yr by TheClient

I took a couple of photos. I can;t work out whether the prong on the right is damaged or has a large build-up of stuff....

And not sure about the cable regards coolant migration, as the wires are exposed I assume you mean inside the sheaths?

The RH probe looks contaminated or damaged - try gently running a screwdriver down the flutes to see which. That is very likely to be the cause of your intermittent issue.

If cleaning the probes does not work, it is a very easy DIY job to replace the EV tank.

The probe connector looks OK to me.

Edited 26 March, 20251 yr by Warrior193
added information

Yeah I think there's a lot of contamination on the right. Try cleaning it. But there is some on the left as well. If it comes back. You'll have to go looking for the source contamination.

The tanks leak in the recess where the connector plugs in. Best to remove connector when hot to check. Spray some soapy water, check for bubbling.

The electrical connectors have ring seals in them. So once plugged in if you introduce pressurised water or steam, it gets pushed into the cable strands and along the cable.. the cable sheath acts as a pipe.

Edited 26 March, 20251 yr by TheClient

Might be a daft question, but if I'm scraping lots of stuff off the prongs, won't it possibly then cause blockages elsewhere?

I was looking for the replacement for the tank, found one https://skodapartsdirect.co.uk/product/skoda-octavia-2013-2025-coolant-tank/ but it isn't clear if the sensor (prongs) is included in it or not.

This one comes with the sensor, but no cap.

https://www.autodoc.co.uk/ridex/14738702#coolant-expansion-tank

3 hours ago, djkarlos said:

Might be a daft question, but if I'm scraping lots of stuff off the prongs, won't it possibly then cause blockages elsewhere?

I was looking for the replacement for the tank, found one https://skodapartsdirect.co.uk/product/skoda-octavia-2013-2025-coolant-tank/ but it isn't clear if the sensor (prongs) is included in it or not.

In all the EV tanks I've seen, the probe section is an integral part of the tank - the wiring connector plugs into the receptacle on the tank.

13 hours ago, Warrior193 said:

In all the EV tanks I've seen, the probe section is an integral part of the tank - the wiring connector plugs into the receptacle on the tank.

Yes, exactly. To the op I would just de mount the tank, drain it to a container clean prongs and replace original coolant. I have a feeling you've got oil contamination so the problems going to recur.

The contamination looked more like scaling to me, I'd have expected to see noticeable discolouration of the coolant if oil was present.

If, on inspection, it is found to be caused by scale, I would strongly recommend a full coolant change.

20 minutes ago, Warrior193 said:

The contamination looked more like scaling to me, I'd have expected to see noticeable discolouration of the coolant if oil was present.

If, on inspection, it is found to be caused by scale, I would strongly recommend a full coolant change.

I agree with scaling. Perhaps been topped up with concentrate and tap water instead of distilled. Had same problem on my MK2 so bought another tank. Wasn't expensive but when it arrived the problem stopped so never got round to fitting it. Might be worth a coolant flush if the scaling is bad.

Alasdair

A thought occurs, it would be worth first just unclipping the sensor plug, removing then refitting - it might be sufficient to clean the connections if that was causing the problem.

I did a run to Gloucester again this morning and I had no warnings appear... The screwdriver scraping might have done the job.

I would still get a new bottle. The fact that there is oxidsation in the coolant tank also probably means you need a coolant flush.