Bad Thermostat P0128

djhedges

Active member
Joined
Dec 28, 2012
Location
Hood River, OR
TDI
2002 Beetle
Our bug threw the engine code P0128. My initial reaction was it's a bad thermostat. I read several forum posts criticizing mechanics for swapping a thermostat without verifying it wasn't the coolant temp sensor. It got me thinking how do you verify it's a bad thermostat?

My biggest indicator was that I could drive the car home with the heat on full and not want to take off a jacket. It wasn't that cold of a day either. I also logged the coolant temp data with a bluetooth ODBII reader and graphed the results.
https://docs.google.com/spreadsheets/d/1mtmV44TQO5UZyrp7oTAHY4ssI-ynyk_Ul2YIZyi2RUk/edit?usp=sharing

You can see the coolant temperature peaked at ~72C on the interstate while driving home with the old thermostat.


The new thermostat reached ~90C on interstate. I believe the thermostat opens at 87C. Also note the lower air intake temps on day I measured the new thermostat.
 
Last edited:

Mountaineer51

Well-known member
Joined
Feb 6, 2007
Location
West Virginia
TDI
2001 Jetta TDI
Our bug threw the engine code P0128. My initial reaction was it's a bad thermostat. I read several forum posts criticizing mechanics for swapping a thermostat without verifying it wasn't the coolant temp sensor. It got me thinking how do you verify it's a bad thermostat?

My biggest indicator was that I could drive the car home with the heat on full and not want to take off a jacket. It wasn't that cold of a day either. I also logged the coolant temp data with a bluetooth ODBII reader and graphed the results.
https://docs.google.com/spreadsheets/d/1mtmV44TQO5UZyrp7oTAHY4ssI-ynyk_Ul2YIZyi2RUk/edit?usp=sharing

You can see the coolant temperature peaked at ~72C on the interstate while driving home with the old thermostat.


The new thermostat reached ~90C on interstate. I believe the thermostat opens at 87C. Also note the lower air intake temps on day I measured the new thermostat.
Usually the best way to tell is the engine won't make it to operating temperature
 

Metal Man

Vendor
Joined
Sep 29, 2001
Location
Sunbury,PA 17801
TDI
1998 NB TDI, 2006 Jetta TDI, 2014 Tiguan gas, , 2019 E Golf X2
I'm not sure what temp the blue coolant light goes off, but it is no where near fully warmed up. It's more of a don't abuse me yet request.
 

jmodge

Top Post Dawg
Joined
Jun 18, 2015
Location
Greenville, MI
TDI
2001 alh Jetta, RC2 w/.205's 5speed daily summer commuter and 2000 alh Jetta 5spd swap, 2" lift, hitch, stage 3 TDtuning w/.216's winter cruiser, 1996 Tacoma ALh
Grab the upper and lower radiator hoses when the car should be at full operating temp, they should be the same temp or somewhat close. I believe the coolant goes out from the stat to the bottom of the radiator, that one will be slightly warmer.
 

jmodge

Top Post Dawg
Joined
Jun 18, 2015
Location
Greenville, MI
TDI
2001 alh Jetta, RC2 w/.205's 5speed daily summer commuter and 2000 alh Jetta 5spd swap, 2" lift, hitch, stage 3 TDtuning w/.216's winter cruiser, 1996 Tacoma ALh
And as it heats, temp will be warmer near the head until the stat opens
 

Metal Man

Vendor
Joined
Sep 29, 2001
Location
Sunbury,PA 17801
TDI
1998 NB TDI, 2006 Jetta TDI, 2014 Tiguan gas, , 2019 E Golf X2
What is the 'blue coolant light' ? :confused:
A NB doesn't have a temp gauge. When the motor is cold the temp light is blue, normal temp the light is out and red if it's hot. It will also flash red if the coolant in low.
 

wonneber

Top Post Dawg
Joined
Oct 12, 2011
Location
Monroe, NY, USA
TDI
2014 Jetta Sportwagon,2003 Jetta 261K Sold but not forgotten
A NB doesn't have a temp gauge. When the motor is cold the temp light is blue, normal temp the light is out and red if it's hot. It will also flash red if the coolant in low.
Interesting.
First time I heard about it.
Thank You. :)
 

Soupinmycoupe

Veteran Member
Joined
Feb 10, 2012
Location
HELLA NorCal
TDI
Manual ALH wagons.
I had the same issue as the OP. Had the same code and replaced the Tstat and housing with a new OEM unit. Cleared the code and then the CEL came back.

This past weekend I replaced the coolant temp sensor and cleared the code again. The CEL hasn’t come back on................
 

Metal Man

Vendor
Joined
Sep 29, 2001
Location
Sunbury,PA 17801
TDI
1998 NB TDI, 2006 Jetta TDI, 2014 Tiguan gas, , 2019 E Golf X2
I had the same issue as the OP. Had the same code and replaced the Tstat and housing with a new OEM unit. Cleared the code and then the CEL came back.

This past weekend I replaced the coolant temp sensor and cleared the code again. The CEL hasn’t come back on................
Yes, the sensors are known to fail and it's cheaper and easier to replace than the T stat.
 
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