p0420 Code is back after replacing CAT


Asked by Nov 10, 2015 at 11:43 AM about the 2001 Toyota Camry LE

Question type: Maintenance & Repair

About 5 months ago at 140K miles I had the downstream CAT replaced with
an aftermarket one because of the P420 Code.  The check engine light
went away.  3 Months ago at 143,700 miles I had the flex pipe crack and
had it immediately replaced with a new one.  Now at 148K Miles I have the
P0420 code back.  I took it back a few days later to the shop to have them
inspect the weld on the flex pipe because it runs a slightly louder than
before.  They said everything checked out.  I'm wondering if the flex pipe
replacement could have any thing to do with the code coming back.  

Can someone help me trouble shoot this?  I know there are several
possible causes and I don't want to just replace parts, I'd like to find root
cause.  Thanks in advance for any insight.

4 Answers


A bad O2 sensor will trigger the code as well. I would suspect that the one on the upstream side is bad. If you want to pay to have them check the sensors with a scope, that's an option. You will still be stuck paying for labor. The unfortunate part is, you can't figure out what's bad until you replace it. Unless you pay for the tests.

The fact that the engine sounds "louder" than usual tells me you need a second opinion and a full inspection of the exhaust system on this vehicle. Also, it would be nice to hook up a scan tool which is capable of displaying "Live Data" such as O2 sensor function. IMO...after market parts...especially...CAT's.....are not my first choice when replacing parts. I would be inclined to clear this code...road test he vehicle and see how fast the code comes back. Then ...a scan tool inspection will provide data detailing what was going on just before, during, and after the computer set the code. This diagnostic procedure should lead one to conclude the root cause for this issue.


I did reset the code and it came back on within the first 25 miles. I think I'll take it to a shop and pay $100 to have them monitor live 02 data.

Sounds like a plan. If they decide to road test this vehicle, you might suggest they take the scan tool with them which will allow them to collect DATA when the event occurs.

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