Steady P0420 at 285K miles

2005 with 285K.

My CEL first came on for a P0420 code at 116K miles but after clearing it would stay off for a few weeks. Now at 285K the CEL will illuminate on the first drive cycle after the code is cleared. Since my car was due for a 300K mile service, I changed some of the easy things and also decided to do a mini overhaul. Here's what I've done so far:

1. Changed spark plugs
2. Changed PCV valve
3. Changed oil
4. Changed 12V battery
5. Changed traction battery
6. Changed shocks/struts (with springs installed)
7. Changed left wheel bearing
8. Added frame brace
9. Changed catalytic converter (Direct fit aftermarket)
10. Changed downstream O2 sensor with an El Cheapo E-bay sensor

Before I get into the P0420 issue, it's worthwhile to mention that if you're planning to replace your traction battery, replacing the rear shocks only requires the removal eight more nuts total and four of them may require removing mounting brackets for the battery. The hardest part when changing the traction battery or rear shocks is removing everything that's in the way. The rear shocks only added a total of 30 mins to the job.

Onto the P0420 code. Prior to replacing the CAT and O2 sensor, the CEL would illuminate on the next drive cycle. After changing these parts, it doesn't come on for quite some time (~7 drive cycles) but the SGII says that a P0420 code is pending. I initially elected not to change the upstream sensor because it looks to be a PITA but I put one on order yesterday and will change it this weekend.

Worthy to mention that I smell exhaust when I idle or back up, which leads me to believe that the gasket which seals the manifold section to the CAT section may be leaking. However, the springs are compressed quite a bit so it doesn't appear that tightening them any further would provide a better seal.

What information can be gathered from this? The CEL went from illuminating on the first drive cycle to illuminating at approximately 7 drive cycles. In other words, the problem seems to have been improved but not fully eradicated. I should also mention the a P0420 code is pending but not set.

 

1. Yes, I agree. The damn thing was too big. It fit on the manifold side just fine but the CAT side just seemed way too big. I would have thought it would be the opposite.

2. Yes. It's an AF sensor, not an oxygen sensor.

I don't understand why a code would be pending as opposed to set. It seems to muddy the water. Either it's good or it isn't. Does it mean anything in this case?

 

Ok, thanks. If I interpreted your post correctly, that means the work I did helped a little but I'm not quite there and hopefully the new gasket or AF sensor will fully solve the P0420 code.

One additional question. Will a pending P0420 fail the car for an emissions test?