Hello,

First I'd like to thanks the folks on this great forum (especially VVTiBob and mandyfig) for providing me with assistance in resolving an issue I was having with my car.

For months my '03 with 90K miles was throwing +" P0420 code Catalyst System Efficiency Below Threshold (Bank 1) " errors. My local dealer and a respected repair shop stated it's the catalytic and must be replaced (at great expense!) I decided to first replace the driver side
O2 sensors one at a time. I replaced the upstream first with no change. I then replaced the downstream and it fixed the problem. It's been 4 months now and almost 4k miles. It was a bit of a problem changing the downstream sensor as the wires go thru the cabin. You must pull back the carpet to access the plug.

Thanks again for your help!

 

The post oxygen sensor is the one that monitors the cat. Glad you got it solved.