I am pulling CEL's P1131 & P0421: O2 Sensor before driver's side Cat - Internal Resistance too High, Warm Up Catalyst Bank 1 Efficiency Below Threshold - Intermittent, respectively.

I've recently replaced deteriorated rubber vacuum hosing and checked the corrugated PVC hosing for leaks without spotting any trouble areas. Replacing the hosing increased performance noticeably initially but car is again running a bit sluggish. I've attached photos of my work and the related VAG COM info.

Can I pass the "Readiness" tests and still need to replace the O2 Sensor and Cat?

 

The "threshold" issue can be programmed out. That will probably take care of your other issue. See your dealer or a very good indy.

Bob

 

Pulled Sensor 1 from Bank 2 (pre Catalytic Converter, driver's side) and it looked pretty bad off. The electrical male/female connections were pretty dirty too which could have contributing to the intermittent signal. Ended up denting the Sensor canister and had to replace the guy regardless of the wear and tear. Went on Ebay and grabbed two pre-cat converters with connection hubs for $160.

I will post some pics for anybody looking to bust the 02 sensor repair on their own. Next steps: breaking out the channel locks for MAF cleaning, Bank 1 sensor 1 02 sensor, clear the CEL's and pray that the Catalysts aren't boned.

 

I've read many posts about the C5 Engine Control Unit (ECU) software update. Sounds like the update can reduce the Check Engine Light (CEL's) codes thrown but my car is/was not running well.

I think I can alter the threshold using the VAG COM software but I'll have to do some more reading.

(Sorry for spelling out the acronyms but it can really help an individual using the forum search function.)

 

The threshold limit changes only gets rid of the code, it does not change how your car runs. If it was running like crap, then you are on the right path (maybe) to recovery.

Good luck,

Bob

 

I now have smoke streaming out of the engine compartment which seems to be originating somewhere beneath the intake hosing just past the Mass Airflow Sensor.

I just changed the O2 Sensor driver side pre Cat and attempted to do the same on the passenger side but after a great deal of unsuccessful torquing decided to give up for the time being.

The smoke smells like exhaust but I am not certain. Could this be a fouled Cat? Could I have detached or punctured a hose/exhaust in my futile attempts to remove the passenger side 02 Sensor? Any help would be appreciated.

 

Did you get your Pre-cat replacements from Ebay welded in? Im skeptical about that to begin with, but warm-up catalyst specifically refers to the pre-cats. I would just replace the entire system or gut them out if you're having smoking problems. Those little guys can easily get clogged and deflect the flow of your exhaust and we all know how close that it is to your turbos exhaust turbine. Just looking at the simple possibilities.

 

I attached a few photos of the Front 02 Sensor removal process and will provide a brief outline below.

Bank 1 - Sensor 1 - (Passenger's Side - Pre Cat)

1. Remove Intake hose after Mass Airflow Sensor.
2. Remove Vacuum Line attached to One Way Check Valve. This is a J shaped rubber attachment which runs a plastic hose to the Driver's side One Way Check Valve (looks like a bell one would attach to a bicycle) and Secondary Air Injection Solenoid. The O2 Sensor in question is located beneath and adjacent to the One Way Check Valve.
3. Remove the two bolts that hold in place the plastic housing to which the various sensor connectors are attached against the firewall (I think it's the firewall). Slide the plastic harness toward the center of the engine compartment carefully.
4. If necessary move the three hoses that run through a plastic router beneath the sensor harness' location prior to step 3.
5. Break out your sensor wrench attachment and slide the O2 Sensor cable through the attachment's slot.
6. Attach your wrench extensions to the attachment. I used a 24'' extension without much luck. I eventually added a 6 inch extension that gave me the angle and leverage I needed to move the sensor.
7. I ended up getting the sensor loose while the car was running. I tried heating the car and stopping the engine but could not get the bolt to budge. Torquing while running the engine was the only way I could pull it off.
8. Once the bolt comes loose let the car cool and remove the sensor by hand.
9. Remove the old sensor connection from the female ECU connection. I cut the old cable sensor cable to aid the removal of the sensor from the engine compartment.
10. Screw in your new sensor and connect the male attachment to the female ECU connection after finding a suitable routing scheme for your new sensor wires. Zip tie the excess cable appropriately.
11. Reattach and replace items removed for accessibility purposes.

Bank 2 Sensor 1 - Driver's Side - Pre Cat
1. Remove coolant expansion tank and carefully place out of harm's way.
2. Remove Vacuum Line attached to One Way Check Valve. This is a J shaped rubber attachment which runs a plastic hose to the Passenger's side One Way Check Valve and Secondary Air Injection Solenoid. The 02 Sensor in question is located beneath and adjacent to the One Way Check Valve.
3. Follow steps 5 through 11 listed in Bank 1 Sensor 1 removal outline.


I was extremely winded after removing the sensors. I ended up cutting the female ECU connection for Bank 1 Sensor 1 after correctly removing Bank 2 Sensor 1 and replacing. Be cautious before cutting wires if you are a putz like this hombre.

 

I attached the wrong photo in the post above. This is a shot of the engine compartment from the front with the wrench attached to Bank 1 Sensor 1 (Passenger Pre-Cat).

 

The smoke running coming from the passenger side of the engine compartment arrived prior to my replacement of the Stock 02 Sensor. The new sensor part numbers are OEM: 078906265M & Bosch: 16 287.

The new sensors fit snugly and I tightened them up a bit with sensor wrench. I didn't go nuts after hand tightening as they are so difficult to remove. After getting the new O2 sensor in the passenger side slot I am not seeing smoke. There is a considerable amount of heat running off the passenger side of the engine compartment but I couldn't see smoke this morning. I am thinking the smoke was due to oil residue burning off as my paw prints were all over the exhaust line. Is this possible?

Photos of the sensors removed are attached. Bank 1 Sensor 1 is showing a great deal of white oxidation while Bank 2 Sensor 1 has black residue all over the tip. Not sure what the white (smoke side)/black residue mean in terms of malfunction indication.