2007 Mercury Mariner 3.0L DTC P0420

[89-LX]

Last night had my Mariner pop up with a P0420 code (Catalyst System Efficiency Below Threshold (Bank 1)) on the way home. The engine still ran great, no significant difference. About 2 weeks ago, I changed out the spark plugs as they were shot and had a miss fire in OD. After I did the plugs, it ran like crap. Turns out the COP harness on the 1 and 2 cyl had the clip break off. So I tore it apart again, and zip tied the harness to the COP, and it ran great for 2 weeks. I've been doing a lot of reading, and it seems it could be the cat itself, or the upstream O2 sensor.

Before I start tearing into it, I just wanted to get an idea of where to look and diagnose before hand, if anyone has access to the flow chart for the diagnostic for this code. Its a 2007 Mercury Mariner Premium with the 3.0L. Miles are 121,000. Thanks.

 

[Detroit Gearbox]

P0420 is usually a bad cat.
I suspect the vehicle ran very rich at one time?

 

[89-LX]

P0420 is usually a bad cat.
I suspect the vehicle ran very rich at one time?

My only guess is when I ran it for a few minutes after the plug swap and the COP wasn't firing correctly on the 1 and 2 cyl. Just as a base line to start, I unplugged the battery to reset the codes, and the code is gone for now, but I've only driven it for 11 miles.

I wouldn't suspect that a few minutes of running rich would kill a cat that fast, am I correct? Long term, as in days and weeks I'd say yes, but not 5-10 minutes worth.