p0128 code on 2004 TDI (BEW Engine)

I've been fighting this code since I bought the car a year ago. It runs fine, but the code is annoying me that I can't resolve the issue.

I first changed the Coolant Temp Sensor. A few hundred miles later, the same code came back on.

Months later, I changed both the thermostat and again a new Temp Sensor (both from the dealer). Again, I get the same P0128 code.

I then changed the outside temp sensor. Same story.

Back to the internet, and I came across a forum discussing repinning the connector to the sensor, as the wires get brittle and cause issues. I've visually checked the connector in the past it it looked fine, but today I took the connector off, untaped the wires and discovered the previous owner already had this repinned. (must have been going on for some time).

My questions to the group is can anyone verify which wires go to which pin numbers? I could be barking up the wrong tree, but since I didn't do the repin, I have no way to tell if they did it correctly.

I traced the new wires back to the original and this is how it is currently pinned:

#1 - Pink wire
#2 - Brown/White
#3 - Blue/White
#4 - Brown/Blue

By the way, the temp readout on the dash seems to be steady and always reads 190 after 5-10 minutes.

Thanks in advance for any help, thoughts our ideas.

FYI, It's an 2004 Jetta TDI BEW engine with 140K. I put about 25K on it this past year with no issue other than bad fuel once.

I just consulted the wiring diagram in my Bentley for an '04 BEW Jetta. Here is what it listed for the connections to the coolant sensor (located in the coolant flange on the driver's side of the cylinder head). This is a dual (two sensors in one) sensor. One is for the engine management and the other is for the dash temp. guage):

Blue/Brown wire from sensor pin #3, goes to pin T60/52 on the ECU.

Brown/Green wire from sensor pin #4, goes to pin 60/53 on the ECU.

Lilac wire from sensor pin #1 to temperature guage in the dash.

Brown/White wire from sensor pin #2 to ground in the sensor wiring harness.

I assume the pin numbers on the sensor match the numbers on the connector to the sensor.

The colors I listed are how they are listed in the Bentley manual. A imiagine the lilac can be the pink you refer to. But you list a color combo that is not mentioned for one of the wires.

You mention a "blue/white" and Bentley mentiones a "brown/green". That is a discrepancy.

My only guess is that the PO used a piece of scrap wire that was a "blue/white" as a splice so it may be connected properly, but maybe he/she did not solder/splice properly and thus you have an intermittent open. I like to solder the wire connection and then heat shrink tube it (some will disagree that this is a proper connection, but I have NEVER had an issue with it).

--Nate

Nate, I appreciate the quick response.

It looks like my #3 and #4 pins may be reversed. Since it is the same pair that runs to the ECU, I wouldn't think having them switched would matter if the sensor just changes resistance across the pair. Since I have the wires exposed, it's easy enough to switch these two.

Thanks for going the extra and providing the ECU pin #'s. I'll track them back and measure any resistance or short in them. I might just run new wires while I'm at it.

I like to solder and heat shrink my connections as well. A little more work, but always a sure connection.

Thanks, Chris

No problem. I like keeping in practice with reading the wiring diagrams.

You would think that reversing the ECU pins on the temperature sensor would not matter, but I guess it could. At least eliminate that possibility. Let us know if reversing the wires works.

--Nate

I got a little motivated today and started checking my wires. Pins 52 and 53 on the 60 pin connector to the ECU are the correct numbers.

I checked resistance between the ECU pins and the sensor. One of the wires had a slight resistance. I checked several times, cleaned my connections and never never get a constant reading. I cut the wire to to get a better connection, cleaned the ohm meter connectors again and still got readings between 0 and .3k ohms.

I looked for a disconnect, but it appears to be a solid run from the sensor direct to the ECU harness. I'm going to run a pair of new wires, button it up, reset the light and give it a go.

I'll report back. It will probably be a few weeks, as once I reset the light it takes a tank or two before the light would come back on.

I love this forum. Thanks again. Chris

What was the outcome ?

bump....I have the same code

I replaced the Coolant Temp sensor and the Thermostat on mine and code still came back. I ended up buying a can of DeOxit and spraying the HELL out of the connector over a period of a week, using an old CTS to force the Deoxit into the terminals. Did this twice a day for a week. Been six months now and no code.

Oh, BTW, if you try this, remove the PINK rubber seal at the bottom of the connector first, as the Deoxit makes it fall apart. Ask me how I know...

that pink seal keeps the connector waterproof. deoxit will make it swell and prevent the connector from seating in correctly

connectors usually don