Bad Temp Sensor?

LadiesMan217

Veteran Member
Joined
Feb 17, 2009
Location
Springfield, MO
TDI
2005 GLS TDI
Hey volks, my CEL came on this morning. The code scanner came back with P0128. Now i just had my t stat replaced about 3 mo ago by "vwjettadsl". The temp gauge seems to work fine. From what ive found on the forum it seems that it could point to a bad temp sensor which i believe i read somewhere on here to be rather faulty. Could anyone shed some light on what i should do about this?
 

Jeff Heyerman

Veteran Member
Joined
Apr 18, 2007
Location
Oakville, ON
TDI
2006 Jetta Wagon (Mk IV)
Well I just went through this. If you have VCDS drive around with someone monitoring the group 00. The coolant "temp" is on that line. It goes from a high to low value as coolant gets hot. At operating temperature it should be about "74" (74 what I have no idea). Mine was fine most of the time, but then jumped erratically on occasion (3 times in 20 minute drive). Also monitor group 01, which gives coolant temp in C - should be just shy of 90C

When you get home, check coolant temp with a meat thermometer (open cap slowly). Should be 87C or so. If coolant is too cool, then prob code is from t-stat not working properly. But much more likely sensor is bad.

The one I just replaced in our 2002 was a black one - apparently notorious for failing. Also for replacement buy from VW, apparently aftermarket ones also let out the ghost quite quickly.

Cheers.
Jeff
 

LadiesMan217

Veteran Member
Joined
Feb 17, 2009
Location
Springfield, MO
TDI
2005 GLS TDI
Well I just went through this. If you have VCDS drive around with someone monitoring the group 00. The coolant "temp" is on that line. It goes from a high to low value as coolant gets hot. At operating temperature it should be about "74" (74 what I have no idea). Mine was fine most of the time, but then jumped erratically on occasion (3 times in 20 minute drive). Also monitor group 01, which gives coolant temp in C - should be just shy of 90C

When you get home, check coolant temp with a meat thermometer (open cap slowly). Should be 87C or so. If coolant is too cool, then prob code is from t-stat not working properly. But much more likely sensor is bad.

The one I just replaced in our 2002 was a black one - apparently notorious for failing. Also for replacement buy from VW, apparently aftermarket ones also let out the ghost quite quickly.

Cheers.
Jeff

Thanks, unfortunatly i dont have the VCDS. I may just have to try the meat thermo method sometime. Could a bad sensor affect mpg? So far after i cleared the code it hasnt come back. Im definately thinking its a bad sensor. Will id parts have replacements?
 

LadiesMan217

Veteran Member
Joined
Feb 17, 2009
Location
Springfield, MO
TDI
2005 GLS TDI
Well so far the code hasnt come back but i just filled up today and its only 30 mpg. I havent been that low since winter. Should i just replace it? Is there a how to on here about it and where's the best/cheapest place to purchase one from? thanks guy
 

TonyJetta

Top Post Dawg
Joined
Sep 15, 2005
Location
Tucson, Az
TDI
'15 Jetta TDI SE / '06 Jetta TDI DSG Pkg0 / '96 Passat TDI
Some preferred sources:
1stvwparts.com - Auburn Wa VW.
idparts.com
worldimpex.com
boraparts.com

Tony
 

LadiesMan217

Veteran Member
Joined
Feb 17, 2009
Location
Springfield, MO
TDI
2005 GLS TDI
Some preferred sources:
1stvwparts.com - Auburn Wa VW.
idparts.com
worldimpex.com
boraparts.com

Tony


Great, thanks. I found an old how-to by "tkindler" so i'll pick up the green sensor and take a crack at it. One question though, my temp gauge has always been stable, if the sensor was really bad wouldnt it be all over the place?
 
Last edited:

TonyJetta

Top Post Dawg
Joined
Sep 15, 2005
Location
Tucson, Az
TDI
'15 Jetta TDI SE / '06 Jetta TDI DSG Pkg0 / '96 Passat TDI
I'm pretty sure the '05 is like my '06, in that there are 2 temp sensors. I had a similar issue and had to replace the one near the thermostat.

Tony
 

TonyJetta

Top Post Dawg
Joined
Sep 15, 2005
Location
Tucson, Az
TDI
'15 Jetta TDI SE / '06 Jetta TDI DSG Pkg0 / '96 Passat TDI
Mine is the a4 design from early 05, before the design change..
If it's a PD, then it's likely similar...2 temps sensors. One at the back of the head, PITA to replace, and one at the lower radiator hose, where it goes into the thermostat, easy to replace. The one in the lower radiator hose is the one I had to replace.

Tony
 
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