Need help with Code P0420

tlocus · October 16, 2005

Hi,

My truck just threw this code today.

I was wondering what to look for or do

Scan tool says:

P0420 catalyst efficiency below threshold (Bank 1)

It had 2 entries of this code (P0420)

t

GM-Tech · October 16, 2005

Bad catalytic converter. Nothing else can cause that DTC. Keep in mind the cats are warranted for 8/80.

tlocus · October 28, 2005

Thanks for the reply.

Is it possible that the cat 'sensor' is bad.

Where can I find (on the truck) this sensor?

8/80 is 8 years or 80,000 miles (whichever comes first), right?

t

GM-Tech · October 28, 2005

Is it possible that the cat 'sensor' is bad.

Yes, but it would set a different code. Never seen anything but a bad cat set this code, and I've seen a lot of 'em. The o2 for monitoring the cat is the one behind it, but it uses a combination of readings on the front o2 and the rear o2 to determine if the cat's efficiency is below the allowed threshold.

Yes, 8/80 means 8 years, 80,000 miles, whichever comes first.

tlocus · November 2, 2005

Hi GM-Tech,

I have one more question

I was looking around for a catalytic conv. I'm thinking about installing it myself.

I found a direct replacement (basically the whole 'Y' assembly made by Walker for about $350 (lowest price I found) at Parks

When I was at the auto parts store, a parts rep (deals with cats also) gave me a lengthy but good info on cats.

He also said something about finding out first why the cat went bad.

Whatever made the cat go bad this time would probably ruin the new one...

Like a tune up etc...

I did just replace the plugs and wires. And I guess while I'm at it, I'll replace all 4 O2 sensors.

What else would you recommend I check or do before i install the new cat?

Currently I have 145000 on the clock

Right now with the bad cat, It doesn't seem to get any hotter than the other(pass. side) or suffer from any performance problem...except for the sticking butterfly valve(when you first push on gas pedal)

Thank you very much!

thomas

Rookie · November 2, 2005

Just food for thought. I had this same code on my Yukon. It ended up being the rear O2 sensor. Now what was causing it was an exhaust leak at the flange right behind the cat. I had put a flomaster on and the flang bolt came loose causeing it to suck air. I fixed it and the code went bye, bye.

Just my .02