[YakRuski]

Hey all, I had my 05 Passat 1.8t for about 1.5 years. Car only has 110k miles on it as of today. I did a lot of work on it both regular maintenance and repairs (normal fluid flushes, full synthetic oil changes at max 5k but I aim 3k. Also had cylinder head resurfaced and I changed the head gasket, don't worry, I torqued the headbolts to spec, with new headbolts.)

HOWEVER, I've had one problem just plaguing me ever since I bought it. 99% of the time I had a CEL for the upper O2 sensor, with the PO130 code specifically. Occasionally it would throw a code for inefficient CAT, but it was rare and after resetting the code and driving for a while, it would only come back after multiple months. The thing is that I have replaced both O2 sensors twice, and I am getting live data via the OBD scanner showing that there is data coming in from the sensors. I don't notice a lack of power; however, I do notice that it idles ever so slightly roughly. I only put about 13k miles on it (I work from home) I currently have both O2 sensors uninstalled, so if there's a way for me to test them in this manner, please let me know.

If anyone needs photos, additional info, anything to help me figure out what is the cause of this problem, I am more than happy to provide said info. Thanks!

 

[PZ]

I would start with checking the ECU side connectors for the O2 sensor. Pull back the rubber boot and look for cracked insulation or corrosion. Then check the wires back to the harness for any damage.

 

[YakRuski]

Gotchya. I don't know what pins on the connector goes to the o2 sensor. I have ALLDATA for my car but I'm lost in terms of the wiring diagrams and whatnot. Also, I did find some ECU's on ebay for my car around $40 bucks. Assuming it is the ECU, what should I look out for when purchasing one?

 

[PZ]

I doubt it's the ECU. They can not be swapped without either the immobilizer being turned off or having the matching instrument cluster swapped too.

 

[YakRuski]

Alright. I checked the ECU side connectors for the o2 sensor, didn't see anything odd. I checked the plug that the sensor plugs into, nothing odd either. I took a multimeter, and measured the appropriate pins on the ecu connector to the appropriate terminals on the port/connector for the o2 sensor. All have continuity, no notable resistance. No wires are shorted to each other, and no wires are shorted to battery ground or battery positive. I also checked the fuses and none of them seem burnt out (it's the one that is on the side of the wheel, not the fuses/relays right above the brake and gas pedals.)

 

[PZ]

Check the grounds, everything from next to the battery, behind the coolant tank, and under the driver's side kick panel.

 

[YakRuski]

Will do tomorrow. Once done I'll update.

 

[YakRuski]

So far, I still haven't noticed anything odd. With other grounds the harness doesn't seem to be shorted to ground. I did notice that when the ECU is plugged into it's harness then on some pins they will connect to a ground, but when ECU is unplugged nothing. One thing that I probably should've mentioned earlier is that I am using aftermarket sensor for the upper o2 sensor. It is a wideband (I think it's called that) sensor, and the two times I replaced it they were both aftermarket. Is there a way for me to test the sensor to see if it is bad?

 

[PZ]

The testing should always be done with the ECU plugged in. I've stuck with Bosch sensors, I don't know if the aftermarket ones are good or not. The sensor should have a heating circuit as well as the sensor circuit and can be tested with an Ohm meter. I don't know the values, but they might be on this site.

 

[YakRuski]

Gotcha. Should I have the engine running or just ignition set to run but with engine off when testing?

 

[PZ]

I expect it should be running. I know the VCDS software measures the date while running, but you might need to measure the voltages at the harness side to see what is wrong.