Temp sensor error

tomcourry

New member
Joined
Jun 24, 2018
Location
San Rafael, California
TDI
1.9L
I have a 02 Golf TDI that is giving me a temp sensor code. I have replaced the sensor. Pin 1 of the harness is not showing +5vdc. The harness does not show an open. I think it might be the relay but don't which one. Any ideas would be appreciated. Thanks, Tom
 

tomcourry

New member
Joined
Jun 24, 2018
Location
San Rafael, California
TDI
1.9L
Temp sensor error code P0128

VEHICLE ISSUE #1: P0128 CODE
TECH REPORT: CHECK COOLANT REGULATOR.
IT DOES SAY THAT THE PROBABLE CAUSE IS COOLANT THERMOSTAT/VALVE

I have already replaced the sensor. Pin 1 should show +5vdc but shows Ovdc
I did not find an open in the harness and do not know where to look next. Could it be a relay and if so which one. Any help would be appreciated.
Thanks, Tom
 

AndyBees

Top Post Dawg
Joined
May 27, 2003
Location
Southeast Kentucky
TDI
Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
Tom, I'll take a look at my electric schematics later this evening/night and post my findings. I've been through them numerous times as I installed an 02 ALH in my Vanagon. But, right now, I don't think the CTS info goes through a relay, to or from the sensor.
 

Ol'Rattler

Top Post Dawg
Joined
Jul 3, 2007
Location
PNA
TDI
2006 BRM Jetta
Can you monitor the measuring block for the CTS with your scanner? Did the new and old sensors show the same resistance at room temperature? No, there would not be a relay involved. Sensor data goes directly to the ECU.

Just for reference:
https://www.samarins.com/diagnose/p0128.html

The DTC does not show up in the Ross Tech data base so I would assume that it is a generic OBD2 code and the trouble shooting directions I posted are most likely still valid even though VW is not specifically referred to.
 
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AndyBees

Top Post Dawg
Joined
May 27, 2003
Location
Southeast Kentucky
TDI
Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
OP, I believe Ol'Rattler has got you covered! And, I found the same explanation for the DTC only on another web site!
 

Ol'Rattler

Top Post Dawg
Joined
Jul 3, 2007
Location
PNA
TDI
2006 BRM Jetta
Well sure Andy, hopefully on the right track. If the ECU does not show CTS data then checking the CTS wiring between the CTS and the ECU for continuity, shorts and opens would be where I would go next.

To the OP. If you check continuity to the ECU with a light or multimeter (Ohms), unplug the ECU connector so that you don't fry the ECU. Sending voltage down a data line and into the ECU can cause some expensive damage. In fact to do those 3 checks on the wiring with any certainty you need unplug the connectors at the CTS and the ECU.

Continuity test in a nutshell:
An important application is the continuity test of a bundle of wires so as to find the two ends belonging to a particular one of these wires; there will be a negligible resistance between the "right" ends, and only between the "right" ends.

The last part is significant because you could assume continuity because you get a continuity indication when in reality you have shorts to ground.
 
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