# P2181 - Coolant Malfunction...Coolant temperature sensor



## clemsongt (Feb 10, 2009)

I have had the P2181 MIL for a while now, and have yet to figure out what the problem is. The options that I have seen as a possibility are: low coolant level, water pump, thermostat, fan switches, coolant temperature sensor (CTS), and coolant level sensor. Since my car has not overheated (needle stays right in the middle), both fans run at idle, and my coolant level is fine, I have looked not looked at the water pump or thermostat. 

I cannot remove or inspect the coolant level sensor because it is built into the reservoir tank. If anyone has any suggestions on how to test it, I would appreciate it.

I have not been able to find much information on the CTS for our engines, I believe the part number is 06A919501A for our car, but I cannot figure out where it is on our car. This thread says that it is just under the fuel pump on the transverse mounted A3 2.0T. I assume that means it is up against the firewall for me. Even with this, I cannot seem to figure out where it is. If anyone can give a little guidance (a picture is worth a 1000 words here), I would appreciate it.

Any other suggestions as to the P2181 problem would be greatly appreciated. I am also going to post this on Audizine.

BTW...the 2.0T FAQ (along with a lot of stickies on this site) are full of links that go nowhere...


EDIT WITH MY SOLUTION:
I am unsure what the exact problem was that caused my P2181 MIL to appear. During an oil change, I noticed a white powerdery residue on the bottom of the crank pulley housing seen here. I eventually figured out that this was dried coolant. From the location, the only logical explanation was that it was a slight water pump leak. Since the leak was so small that I hadn't even noticed a loss in the reservoir, I let it go for a while. 
When winter came, I realized that it was taking a while to reach temp. This is indicative of a stuck thermostat. 
During the following summer, when I finally got time to work on it, I did an entire timing belt change and replaced the water pump and thermostat while I was at it. The code has not come back. Whether it was tstat or water pump leak, I am unsure, but I am sure it was one or the other. If I had to guess, I would guess it was the thermostat given how slight the leak was (added a few ounces of water once in a years time).


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## Krieger (May 5, 2009)

i have been told by ECS tuning that one is on the oil filter housing, and the other is on the lower rad line.

when you order the sensors, order the needed clips too. I am in the process of hunting these suckers down, but tbh, they are ****ing allusive. lmao

my car is just overheating like no tomorrow, but no MIL or anything... my fans just run for over 5 minutes when i shut her down now, plus my AC stopped working to avoid overheating the engine... when your thermometer hits 109 degrees and you see like 90% humidity, that SUCKS!

anyways, ill be looking tomorrow hopefully, maybe thursday. If i find them, ill take pics and do a quick how to.


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## clemsongt (Feb 10, 2009)

Well I stumbled around under my hood and was able to find the sending unit. It is on the rear of the block just above the catalytic converter on the passenger's side. [Location is for longitudinally mounted 2.0T's...a transverse mounted 2.0T would probably be on the rear of the block on the driver's side.]

I took a picture of what I thought was the sending unit:










I was then able to work ETKA well enough to confirm this:

This is a picture showing the part mounted to the block that holds the sending unit (CTS):








This is the illustration showing where the 'water connection' is in the grand scheme of things:








Less zoomed in than the picture above:









I guess if/when I figure out what is causing my P2181 error, I will relay that information back here just to add more info to the archives.


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## Krieger (May 5, 2009)

so is it just the one sensor? ECS told me there where two, one on the oil filter housing, and one on the lower rad line...

grrrr.


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## VWAUDITEK (Dec 26, 2006)

Thermostat....I had one of these on a 06' recently,tried the CTS first (takes 10 miutes to change)ended uo beingthe TSTAT.


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## Krieger (May 5, 2009)

swapped my coolant temp sensor and it did not fix the issue. All it managed to do was spill coolant all over my parking spot and and get coolant all over my transmission. awesome.

so, next up is the thermostat. tips as to doing it?

might as well do a coolant flush and refill to get out all the burned up coolant since ill be redoing all the coolant sensors now anyways.


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## mista808 (Apr 30, 2000)

I'm having the same code with my 06 a3 right now.. Went the least expensive route and changed the ect sensor first. 

First, my temp gauge stopped working, then started working again. Then stopped with a CEL.. CEL still on then gauge started working again... Changed the ect sensor, and cleared the CEL and gauge worked once again.. Today, the gauged stopped working with no CEL.... So now I'm gonna change out thermostat and hope for the best... 

I got a friend that works at a Audi/vw shop and said every time a car comes in with a p2181, he changes out the ect sensor and thermostat together... Anyone confirm this fix?


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## puttertech (Aug 4, 2010)

mista808 said:


> I'm having the same code with my 06 a3 right now.. Went the least expensive route and changed the ect sensor first.
> 
> First, my temp gauge stopped working, then started working again. Then stopped with a CEL.. CEL still on then gauge started working again... Changed the ect sensor, and cleared the CEL and gauge worked once again.. Today, the gauged stopped working with no CEL.... So now I'm gonna change out thermostat and hope for the best...
> 
> I got a friend that works at a Audi/vw shop and said every time a car comes in with a p2181, he changes out the ect sensor and thermostat together... Anyone confirm this fix?


 That's what i was told today, going to give it a try in the morning, hate that you can't just buy a thermostat like any other car, you have to replace the housing and all. Got the same code two days ago, i've got no heat and my temps are off by about 10 to 20 degrees so i figured it's the thermo.


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## sma3 (Jan 24, 2006)

What was the final diagnosis? I'm having the same issue described. Temp gauge died on the way to work a couple months ago. Came back to life after a few days. Died yesterday. CEL came on this time. Now the temp gauge is working again but the CEL is still on (probably cause the codes are still in memory). Of note, the temp gauge seems to be reading slightly lower than normal. Needle stays left of center on the cold side now. I remembered it being straight up and down previously.

Should I replace the CTS or the Thermostat? Both??


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## clemsongt (Feb 10, 2009)

sma3 said:


> What was the final diagnosis? I'm having the same issue described. Temp gauge died on the way to work a couple months ago. Came back to life after a few days. Died yesterday. CEL came on this time. Now the temp gauge is working again but the CEL is still on (probably cause the codes are still in memory). Of note, the temp gauge seems to be reading slightly lower than normal. Needle stays left of center on the cold side now. I remembered it being straight up and down previously.
> 
> Should I replace the CTS or the Thermostat? Both??


Final diagnosis for me is still pending. I think it is a combination of failing waterpump and/or thermostat. I have discovered a small leak leading me to think waterpump, and it takes a long time for my temp to come up leading me to think thermostat. 

My temp gauge, however, has never shown anything that seems to be off. When you say that your temp gauge 'died', I assume you mean that the needle dropped all the way to cold. If that is what happened I would 100% think CTS. If your thermostat was failing that would be a completely separate issue and merely a coincidence.


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## sma3 (Jan 24, 2006)

clemsongt said:


> Final diagnosis for me is still pending. I think it is a combination of failing waterpump and/or thermostat. I have discovered a small leak leading me to think waterpump, and it takes a long time for my temp to come up leading me to think thermostat.
> 
> My temp gauge, however, has never shown anything that seems to be off. When you say that your temp gauge 'died', I assume you mean that the needle dropped all the way to cold. If that is what happened I would 100% think CTS. If your thermostat was failing that would be a completely separate issue and merely a coincidence.


Yep, it dropped to zero. Although the temp gauge seems to be taking longer to warm up and settling just shy of the mid line on warm 100+ degree days. That leads me to believe the tstat is stuck open. I read a post earlier in this tread from VWAUDITEK. He tried the CTS but it didn't fix the problem. Followed up replacing the tstat, which did fix the problem. If the CTS is cheap I'll probably get both done. 

Thanks for the help!!


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## [email protected] USA (May 17, 2007)

VW/Audi is well known to have coolant temp sensor issues. Cheap and easy to change that sensor first (yes i know some in this thread did that just posting for future searchers). I'd replace this first after doing a basic fan check (see below)

The best way to do a coolant temp sensor is first thing in the morning. Despite what any procedures say do NOT open the coolant cap to relieve pressure (assuming you aren't an idiot and the car is actually cold). Doing it with the coolant cap on and in the morning when its cold will basically retain a vacuum on the system and keep it from losing too much coolant. I've done this and the only coolant lost was the thin film on the sensor itself. Not as easy on a4s though since the heater core hoses are the highest point in the car and it is much harder to reach than in the transverse applications.

Normal test procedures for a thermostat stuck closed or a waterpump failure won't help you since they both will restrict flow. For the early 2.0ts waterpumps have been fairly reliable compared to say he 1.8ts. If you are getting some overheating easiest way to tell if it is a waterpump vs a thermostat would be if it overheats while driving but cools while stopped. If it was a thermostat stuck closed it would overheat and stay hot. With the waterpump failures common to VWs if the thermostat is open at low engine speeds the pump usually will be able to push some coolant and cool the engine, at high engine speeds it slips at the same time producing more heat. A stuck thermostat may cool more at speed due to airflow over the engine itself and overheat when stopped. But again these can confuse each other. In this case do a thermostat first much much easier. If you need to do the waterpump just do the whole timing belt to save time and money.

If your thermostat is stuck open then the car will take longer to heat up but will eventually usually get up to temp. You may notice temps drop significantly at highway speeds if the thermostat is stuck open. 

Fan operation should be easy enough to test, make sure they come on period when the AC is on to verify they at least work. They should come on a minute or two after the car reaches full temp. Sit with your foot on the gas at about 3K rpm and usually it will get up to temp and kick the fans on within 2-3 minutes. Often times you need to let off the gas and wait 30 seconds or so since if the thermostat is working or stuck open and the waterpump is working at 3K rpm it will move coolant through the radiator even at a standstill to keep the fans off until flow is reduced at idle and it heat soaks a bit then the fans should come on. 

There in some applications is a secondary fan temp switch on the lower radiator hose. I'm blanking right now if all have them or just the A4s. IF the fans aren't coming on check that first. VW is also know to have had issues with fan control modules in applications that have them. Like when the brilliantly put it under the coolant bottle on the mk3s... car overheats coolant bottle lets off pressure, coolant falls onto fan module and blows fuse making it even harder to cool down.. BRILLIANT!!


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## [email protected] USA (May 17, 2007)

sma3 said:


> Yep, it dropped to zero. Although the temp gauge seems to be taking longer to warm up and settling just shy of the mid line on warm 100+ degree days. That leads me to believe the tstat is stuck open. I read a post earlier in this tread from VWAUDITEK. He tried the CTS but it didn't fix the problem. Followed up replacing the tstat, which did fix the problem. If the CTS is cheap I'll probably get both done.
> 
> Thanks for the help!!



If its taking longer to warm up and staying just below where it is normal then i'm 99% sure your t-stat is stuck open, assuming your fans aren't on constantly but that would be obvious. 

If you do a long highway drive at moderate speed and reasonable rpm. say 55-60 not like 90. you may notice it drop even more and that would pretty much lock in the diagnosis of a t-stat stuck open.


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## sma3 (Jan 24, 2006)

For the record, I had this issue fixed today. Was driving down to San Diego from LA. The temp gauge stopped working. I assumed it was fine but didn't want to risk it. Can't imagine what it would cost to rebuild the engine. I dropped it off at Beach House Imports in Orange County. They replace the tstat and both sensors. Probably not the cheapest place I could have had it done but they were friendly and knowledgeable (more so than some of the dealers I've been to). They also didn't try to charge me an hours labor for the diagnostic work like the dealerships typically try to pull, which I appreciated. Total cost was around $600 out the door including tax, fluids and an oil change. 

Gauge is now pegged dead center. Seems to be running really well.


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## mista808 (Apr 30, 2000)

You say both sensors... Where exactly is the other sensor located? I'm waiting on a new thermostat to come in... Mine also gets cooler when the car is moving, then is straight up and down when stopped.. Guess it's stuck open, better than it being stuck closed...


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## sma3 (Jan 24, 2006)

Not sure of the location of the second sensor since I had them perform the work for me. Sorry.


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## MkVfTw20 (Nov 28, 2011)

anybody ever have the temp sensor under the intake manifold literally explode out of the water pump???


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## 10CSMGTI (Mar 27, 2010)

I have thrown a P2181 code with CEL. Bought a coolant temp sensor, can anyone tell me where it is exactly located? The guy in the parts dept. at VW dealer told me it was on the side of the battery lol, yeah that helps a lot!

Anyway: 2010 GTI 2.0 Tsi

Part number: 06A-919-501-A


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