Code PO420: Bank 1 Cat system Efficency Below Threshold. After hearing a ratttle in my cats I replaced them with the Magnaflows. Now I am getting this code (which I've had going for over a year somewhat recently after the replacement). I havn't bothered addressing this yet because I know it's just an emissions issue but the check engine light is annoying me. They were installed well with no holes in the welding job. The truck runs great otherwise. Decent gas mileage all cylinders haveproper pressure. OKdiscoguy you went with the Magnaflows as well - any problems similar to this? I'm thinking it's time to replace the O2 sensors but usually that's not done till around 100k. Any ideas? Thanks.

 

Is it possible that your muffler, or resonator are partially clogged? It is not just the cats that affect exhaust performance.

 

I would look in front of the cat for a leak, all the way up to the heads.
Is this your only fault code?
That code could also be coming up because of an 02, specificaly the rear 02, so you may have another code coming soon.

 

I just went through this too. Small leak on bank 2. Look for leaks..

 

Catback was replaced about a year prior to the cats. So everything from the exhasut manifold down has been replaced.

 

Thanks for the quick replies guys. I will double check for a leak. Sitting in the drivers side, is bank one the left or right handed side?

 

Yes the only code I have. Had it for over a year.

 

BANK 1 IS THE DRIVERS SIDE.

 

I think Mike just yelled at you....

Check the driver's side manifold for cracks. They can be really tricky to find....

 

For future reference, the cat system in my 2001 D2 is covered by an 8-yr/80k mile warranty. I don't know if that's the case for other model Disco's but might be worth looking into for anyone whose Disco is showing symptoms and falls into that range.