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This afternoon, I left my 2012 VW Beetle 2.0T idle for a few minutes at a gas station, and it showed a hot coolant temperature warning; the sensor was reading 195F. It was an unusually warm day for this time of year (80-85F), though not the warmest it’s ever been put through.

Problem is, the oil temperature gauge read a solid 215-225F at the time, which is normal for that engine. For that reason, I felt comfortable driving it back home (~1.5mi). The oil temperature did not rise after the warning. Fans are also working fine.

About 4 hours later, I decided to check in on the car, only to be greeted with the same warning - though the coolant temperature was lower, at 110F. I started the engine (whose oil temperature was way under 200F), and the coolant temperature reading rose again to 195F in under a minute. (So I think the pump is working fine)

Coolant levels are way above minimum and there’s nothing obviously wrong under the hood. Of note was some bubbling near the coolant reservoir inlet when I went to check the car for the second time - though I did open the cap (with a rag on top, obviously) and it turns out that I could have very well done it bare handed.

I have no idea what this could be (especially since the sensor reads were consistent), so any input at all is appreciated.

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  • Was the fan spinning? Could also be a faulty thermostat.
    – Bart
    Commented Feb 22, 2018 at 9:28
  • Why is 195°F (90°C) reported as hot? That is a perfectly normal temperature for engine coolant.You may start to worry when it passes ~220°F (105°C) but up until that you're fine
    – MadMarky
    Commented Feb 22, 2018 at 15:31
  • I agree that the reported temp isn't too high but then again, I've had overheating cars register a fine temp. I wonder if the gauge and warning light are from the same source on this car.
    – seizethecarp
    Commented Feb 22, 2018 at 15:41
  • @Bart yes, the fan was spinning. The temperature also seemed reasonable, which is why I didn’t immediately blame the sensor.
    – osuka_
    Commented Feb 22, 2018 at 16:48
  • @seizethecarp probably not? They measure temps of different fluids and at times there was a difference of 80F between the two
    – osuka_
    Commented Feb 22, 2018 at 16:50

2 Answers 2

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I assume you have more than one coolant temperature sensor. There probably are two: one for the ECU, another for the gauge. The sensor for the ECU may be creating this warning due to a fault, whereas the sensor for the gauge shows perfectly normal temperatures.

So I would start by looking at how many coolant temperature sensors there are and for what purpose they are used. You may need to replace the coolant temperature sensor for the ECU.

If the fans were spinning, the temperature shouldn't rise unless there's a major mechanical problem with thermostat, radiator, etc.

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  • That's a really good thought - I just got back from the garage and the car still had the light on, but the reading was a chilly 55F. I turned it on and the coolant temperature rose faster than the oil temps. I feel like it could be some other part of the cooling system - what should I check for?
    – osuka_
    Commented Feb 22, 2018 at 19:43
  • I believe there are two sensors as well, if you look at this page, you will see that one of them is a very common issue that causes the problems you describe @osuka_ shopdap.com/blog/vw-audi-20t-tsi-common-problems.html
    – finleyarcher
    Commented Feb 23, 2018 at 3:30
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    VW sensors like to report "your car is screwed" information when they go bad, because it's a "fail safe" design.
    – finleyarcher
    Commented Feb 23, 2018 at 3:31
  • @finleyarcher if anything I can appreciate that - I’d rather have an overzealous car, than one that blows up without warning.
    – osuka_
    Commented Feb 23, 2018 at 3:55
  • @osuka_ the emphasis of the sarcastic "" is that if your sensor reports a warning but is still operating, it hasn't failed. Fail safe designs are necessary, but they also need to work correctly. A good example is brake switches, when they fail, they fail on so your brake lights stay on. If your coolant sensor fails, it should provide a nonstop dangerous temperature signal i.e. 999 degrees. That way you know something is truly wrong with the switch and can just replace it instead of having to troubleshoot your system.
    – finleyarcher
    Commented Feb 23, 2018 at 4:12
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Bubbles in the coolant reservoir? You definitely have a cooling system leak. The hot warning light will quickly warp and permanently damage an engine.

I've read that about 15% of an engine's heat is dissipated through the oil & pan. It's important, but it affects mostly the lower parts of the engine. The sensitive, upper parts of the head are not nearly cooled as much by oil. Don't rely on the oil temperature gauge at the expense of the water temp. gauge.

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  • I do not agree with what you're saying here. The reported temperature is normal and a "hot warning light" itself will not damage anything. The bubbles could point to pressure buildup due to a faulty head gasket but nothing more than that.
    – MadMarky
    Commented Feb 22, 2018 at 15:28
  • @MadMarky I don't understand what you are getting at with this reply. I think your attempt to make a distinction between the hot coolant and the hot coolant light is only adding confusion to the issue. Of course the light itself will not hurt anything; it's the heat that it's registered that will do the damage. Further, the way you downplayed a faulty head gasket is confusing. A bad head gasket is like a $1000 repair right there that if left unchecked will also kill the motor.
    – seizethecarp
    Commented Feb 22, 2018 at 15:41
  • @MadMarky so why would a blown head gasket cause any of this, especially in the absence of other symptoms?
    – osuka_
    Commented Feb 22, 2018 at 16:54
  • Engine oil temps have be +400F before you will see severe oil failure conditions, most engines run nominally right over 200F and coolant on most modern cars is running ~170-220F nominally depending on if the thermostat is open. You are right about the bubbles but you need to edit your answer, you say the light will warp and damage the engine.
    – finleyarcher
    Commented Feb 23, 2018 at 3:36

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