# Coolant Temp Sensor, failure symptoms?



## Tyrolian (Apr 1, 2009)

I have a 2004 1.8T (AWM) Passat that is over heating per the temp sensor. Temp sensors are known to fail. Do they fail with a false hot reading? 
The sensor in my S4 failed with the output intermittently dropping from normal temp to cold then back to normal. No false hot reading.
On this Passat the temp reading goes from cold start up (cold engine), to normal 190F as the thermostat opens. Then after a few more minutes with some load it starts rise to overheating temp. The auxiliary, electric fan never turns on. 
Vag-com codes: 
From Engine computer:
19497 - Powertrain Databus: Implausible Engine Temp. Message from Inst. Panel 
P3041 - 004 - No Signal/Communication - Intermittent
16501 - Engine Coolant Temp. Sensor (G62): Signal too Low 
P0117 - 001 - Upper Limit Exceeded - Intermittent - MIL ON

From Instrument panel computer:
01039 - Coolant Temperature Sensor (G2) 
64-10 - Not Currently Testable - Intermittent
01039 - Coolant Temperature Sensor (G2) 
30-10 - Open or Short to Plus - Intermittent
Possible cause that I can see:
1) faulty temp sensor, engine is not really hot
2) faulty aux fan switch, engine is getting hot
3) flow restriction, partial opening of thermostat or failed pump impeller.
4) blown head gasket (no abnormal idle, or "smoking" exhaust, yet)
Any good troubleshooting advice? How to test sensors or flow?


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## vwfansince95 (Nov 13, 2007)

*Re: Coolant Temp Sensor, failure symptoms? (Tyrolian)*

sometimes the sensor triggers a CEL and sometimes not.. 
just go and purchase/replace it.. they are prone to go bad and are less than $20.00 brand new. it ur case, ide replace the sensor first and see what happens next, but pretty positive thats the route of ur problem. mine was bad but never threw a code or triggered the CEL. it was suspect to me for a long time. i think im gona make it part of general/routine maintenance every couple years now.


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## Tyrolian (Apr 1, 2009)

*Re: Coolant Temp Sensor, failure symptoms? (Tyrolian)*

Update on my coolant temp sensor question. The sensor is correct, the overheat was real. I drained the coolant and pulled the thermostat. I found a piece of the plastic water pump impeller. From the thermostat hole, I could feel the back of the water pump impeller and move it around. If there is a 1.8T troubleshooting advice guide out there it should be noted that this total impeller failure check can be done on the later model 1.8T (AWM).
So the real issue is a an original VW water pump that has failed at 61K on 2004 passat wagon. This is a Family Truckster that is driven mildly. 
I have read about replacement pumps with a metal impeller. Graf makes one, are there other manufacturers? I have read about "risks" of a metal impeller, but being a mechanical engineer by trade, I don't believe most of these comments. A plastic impeller is just cheaper to make. The light weight material along with the smoother casting, most likely avoids any post casting machining to balance the impeller (= manufacturing $ savings). The only potentially real negative comment I can see is if there are one or two companies making a metal impeller version and their bearings and/or seals are poor quality. My old '87 Audi 4KQ used a similar pump design with a metal impeller and I never heard of an impeller coming off the shaft, but I did have seal fail early. I think VW/Audi is just using plastic to reduce costs and does not care much about longevity past the warranty period until the negative feedback starts to affect new car sales. If there were enough complaints about this, they would address it, like the examples of extended warranty on faulty MAF sensors, faulty coil re-calls, coolant temp sensor re-design, etc. 
After owning and maintaining VW/Audi cars for 14 years, they need to address the following consistent poor quality issues before they will win my trust to buy another car from them:
Water pump failures, plastic impeller 
Coolant temp sensor failures
Expansion tank failures due to cracking
ABS controller failures, due to under rated circuity
MAF sensor failures
Instrument cluster center digital displays that fail
Window regulators that fail
Vacuum Vent Valve switches that fall apart (newer all electric versions)
Leaky valve cover gaskets
Leaky cam tensioner seals
When I see these failures on not just one car, but on multiple similar cars of a small sampling, I loose trust.


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## vwfansince95 (Nov 13, 2007)

*Re: Coolant Temp Sensor, failure symptoms? (Tyrolian)*

agreed, owning a VW sometimes can be stressful. the upgraded metal impeller water pumps from VW have replaced the plastic ones. your not alone in this matter. if the plastic impeller water pumps dont go by 60K or so, most people upgrade to the metal impeller pumps anyway when doing the timing belt and other related cooling system parts. 
as for the other items you noted, these are issues 'known' to many of us esp the; coils, MAF, coolant sensor and etc. best bet is to stay tuned to vortex and the threads on here to keep up with updates/developments with our cars. i chime in at times regardless if im having problems or not. and sometimes avoid problems and issues by getting to it first before it occurs. the truth is, as time goes on and our cars get older a little more TLC is required. i guess its the aches/pains/joy of driving these machines that keep us coming back for more. i guess thats where the word enthusiast derives from.. 
either that or we're all work slaves for VWoA


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