culi

Recently my check engine light came on for a couple of days and then off. I scaned it, and it showed code P0420 - Efficiency Below Threshold for Catalyst Sytem Bank 1.

The strange thing is that it came on for a couple of days and then goes away by itself. The scan does show "MIL ON - Intermittent - Confirmed". This has happened twice in the last 3 months or so. I have noticed that it happened on very humid days. It does go away after a couple of days.

Since my car is around 52K miles, so it is outside of the warranty. My next step is check the sensor, but that doubt that the sensor is bad since the light goes off after a couple of days. Maybe a leak in the exhaust system? Anybody else having this issue? Also, which side is bank 1, passenger or driver?

Here is what I found that might help me solve this problem.
P0420 DTC Code - Catalyst System Efficiency Below Threshold (Bank 1)

tenspeed

Usually sensors get slow around 100K miles and changing them increases the MPG's. I would be surprised if it was bad at 50K miles. Try swapping the bank 1 and 2 sensors to see if the code shifts next time you get a CEL. I believe bank 1 is the drivers side. Check all the wires and clean the sensor contacts as long as you're crawling under the car.

There are DIY instructions for testing a sensor on the internet. It involves the sensor in a vice, a torch and a multimeter. Heat the sensor and time how long it takes to change the resistance.

Calimus

I'm having the same error, but I'm at 127.7k right now and it started about 126.2k. It tends to only come on when it's cold in the mornings, is on for a few days or less. I had already figured it for the after cat O2 sensor on bank 1 just due to mileage. I'll be ordering a new sensor in the next month or so and replacing it.

tenspeed

IIRC Snagit said something in another thread about a 100,000 mile emissions warranty. Give the service department a call to find out what is covered.

I found this PDF with a search - http://www3.epa.gov/obd/pubs/420f15035.pdf