As I'm sure many of you know there are millions upon millions of P0420 threads. Many of them because of exhaust mods.
Anyway I did my searching and the only thread I found satisfactory was this one. P0420 Catalyst System Efficiency Below Threshold (Bank 1)?! - Subaru WRX Forum: WRX Forums
As I was driving to work today my cruise control stopped working, the check engine and tcs off light came on. The "cruise" light now blinks steadily at all times. I took it to o'rileys and used the code reader. I got "P0420 Catalyst System efficiency below threshold (bank 1)". There are no mods on the car currently. Should I do like the thread suggested and replace the rear o2 sensor? Car is an 08 wrx.