I am the new owner of a 2004 Volvo S60. Its inspection is due in 5 days and I'm getting the dreaded P0420. I've reset the code but it came back in less than 100 miles. Here's what I can share:

I borrowed an OBDII scanner from AutoZone. The live data shows that the Sensor 1 (upstream) is hovering around 1 volt and Sensor 2 (downstream) is switching between 0.1 volt to 0.9 volt. According to several videos I watched on YouTube, it should be Upstream Sensor that's switching and Downstream Sensor staying more stable. Therefore, I'm assuming that the scanner is not reporting the sensor names correctly.

Since the Downstream Sensoris not switching like the Upstream Sensor, I'm guessing that the CAT is doing its job, and that I may have a problem with one or both of the O2 sensors, but I'm not sure how to verify this. The exhaust does smell funny, (almost like a bad CAT) so I do trust the Downstream Sensor is a little bit more, and am considering replacing the other one.

Here's the scanner graph:


Any suggestions?

 

I have heard that you can sometimes temporarily clean a sensor by pulling it and hitting it with some electronics cleaner spray.


The cleaner, an O2 socket and a trip to autozone to clear the code is a cheap test if you have the time. If the code stays away I would just plan on replacing the O2 sensors in the near future when time and money allows.

Has the car been emissions tested recently? If the cat is going bad I would expect the pollutant emissions to be higher although lazy O2 sensors can cause a similar issue. The fact that the first O2 sensor is switching quickly might mean that it is doing ok but they can also become contaminated which is where the cleaner can sometimes help out. How many miles are on the car?