Recently I got the dreaded P0420 code on my 2003 A4 1.8 indicating poor cat performance. A few people on this forum have stated that most of the time this means the cat is bad, which isn't true. Most of the time this code simply means that an O2 sensor has or is failing, or there's another issue upstream that's causing the fuel/air ratio to be too far out of spec for the cat to effectively scrub the NOx emissions to put them within the specs the downstream O2 sensor is looking for.
I use the Torque light app for most typical diagnostics because it works great with a standard Bluetooth OBDII interface. I also have VCDS but usually only pull that out when I need to get serious. When I monitored the O2 sensors with Torque, it appeared that the upstream sensor was dead because it showed no output voltage. I've had O2 senors go dead in other vehicles with no code so this didn't surprise me.
What did surprise me is when I cut the O2 wires to install a spare 4-wire sensor I had, I discovered what many of you already know - the upstream sensor is a true 5-wire wideband. Two Passat 1.8's I own use the 4-wire and register in Torque fine so I was expecting to see the same thing on my A4.
I ordered and installed a Bosch OEM replacement (part number 17351). Torque still showed nothing for the front sensor (Bank 1 Sensor 1 - B1S1 for the record). The car ran better immediately, had smoother acceleration and on a 30 minute or so test run to reset the cleared fault (clearing the P0420 actually resets several other parameters that need time to reach a readiness state), mileage on the highway improved to an indicated 34 MPG from 28 MPG. Obviously the new sensor was doing its job.
After researching the Torque issue, a few others have also stated that Torque apparently can't read these 5-wire O2 sensors. I sent email to the app developer to ask if the Pro version of the software reads the 5-wire sensor but haven't heard back yet. Some remarks from other users of the app would indicated that it doesn't.
So, it's VCDS for monitoring the 5-wire sensors until I hear if Torque can or will be updated. My Audi is a 12-year-old vehicle so I'm surprised Torque doesn't work with these sensors.
Anyway, here's a good explanation from "Old Guy" of how the sensors and cat work. I would add that his last statement assumes properly working sensors and that other upstream components are working properly-
That works. Or just monitor the B1S2 (downstream) O2 sensor voltage with a Torque app. The A/F mix is constantly being adjusted by the front wideband A/F sensor. If you monitor the front wideband sensor when steady state driving it should pretty much remain at an equivalency ratio of 1 which indicates a normal A/F waveform. This is NOT to be confused with the waveform of a standard (non-wideband O2 sensor). When you get on/off throttle you will see it go lean (1+) or rich (-1).
The cat either stores or releases oxygen based upon the NOx mix entering the catalyst. The rear O2 sensor monitors how well the cat can scrub the NOx gasses. Consequently it should remain stable when you are steady state driving. A reading of .5-.6v with steady state driving would indicate that the cat is doing its job. If the voltage is constantly fluctuating above and below .5v it would indicate that the cat is not storing/releasing O2 as needed to scrub the NOx which would indicate a bad cat.
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