Catalytic Converter Code (P0420)

Over Christmas, my CEL came on. I wasn't in a position to take it to a
dealer, so I checked out my first theory a couple hours (and 100 miles)
later. I'd had my oil changed a few days (and 200 miles) earlier, and
I guessed they didn't put the air filter canister together right. I
checked, and that was a problem - the canister wasn't put back on
correctly. I put it back together and drove it another 70 miles and
the CEL went out after about 2 drive cycles.

A week or so later, the light came on again. This time, I took it to a
local Advance Auto and had the code scanned - P0420 (catalyst
efficiency below threshold). I took it to the dealer where they told
me I needed a new cat - $1100 was the estimate. I figured I'd try
replacing the O2 sensors first, incase they were marginal or starting
to fail w/o throwing the O2 sensor code. The CEL went out by itself
before I put the sensors on...put the sensors on anyway. I drove the
car about 600 miles and the CEL came back on. Checked the code again
and another P0420 showed up. After a couple days and a few short
drives (40 miles or so total) the light went back out again and is
still out.

Anyone else had a similar experience? Any chance the incorrectly
assembled filter canister could have somehow prompted this to happen??


I'm buying an OBDII scanner on Ebay so I can keep it w/ the car and
scan problems on-the-spot for the future, but I'd love to know what the
code was the first time...anyway..

If anyone has thoughts on this, please let me know. If I do put a new
cat on the car, I'm going to put an aftermarket cat....probably a
D.E.C. Anyone have thoughts or opinions on aftermarket cats (good
idea/bad idea, problems, best choices?)

Thanks!

On 22 Jan 2006 19:59:21 -0800, wrote:

>Over Christmas, my CEL came on. I wasn't in a position to take it to a
>dealer, so I checked out my first theory a couple hours (and 100 miles)
>later. I'd had my oil changed a few days (and 200 miles) earlier, and
>I guessed they didn't put the air filter canister together right. I
>checked, and that was a problem - the canister wasn't put back on
>correctly. I put it back together and drove it another 70 miles and
>the CEL went out after about 2 drive cycles.
>
>A week or so later, the light came on again. This time, I took it to a
>local Advance Auto and had the code scanned - P0420 (catalyst
>efficiency below threshold). I took it to the dealer where they told
>me I needed a new cat - $1100 was the estimate. I figured I'd try
>replacing the O2 sensors first, incase they were marginal or starting
>to fail w/o throwing the O2 sensor code. The CEL went out by itself
>before I put the sensors on...put the sensors on anyway. I drove the
>car about 600 miles and the CEL came back on. Checked the code again
>and another P0420 showed up. After a couple days and a few short
>drives (40 miles or so total) the light went back out again and is
>still out.
>
>Anyone else had a similar experience? Any chance the incorrectly
>assembled filter canister could have somehow prompted this to happen??
>
>
>I'm buying an OBDII scanner on Ebay so I can keep it w/ the car and
>scan problems on-the-spot for the future, but I'd love to know what the
>code was the first time...anyway..
>
>If anyone has thoughts on this, please let me know. If I do put a new
>cat on the car, I'm going to put an aftermarket cat....probably a
>D.E.C. Anyone have thoughts or opinions on aftermarket cats (good
>idea/bad idea, problems, best choices?)
>
>Thanks!

You didn't say what car you have, year and model, also if you are in
the US. If in the US and your car is under 8 years and 80K miles,
there is a federal emissions warranty that covers the catalytic
conveter. As long as you haven't caused the failure by modifying the
car the cat should be replaced by the dealer for free under this
warranty even if your regular warranty has already expired.
This code is almost always caused by the catalyst itself, and not the
02 sensors. A bad 02 sensor will set its own code or codes, if all you
got was the P0420 by itself it's most likely the cat.
The loose airbox may have caused a different code, possibly a P0171
lean condition code, but that would have nothing to do with the P0420
or the catalyst. If the codes have not been cleared with a scanner and
the battery has not been disconnected, it should still be possible to
retrieve that first code by scanning for codes stored in the ECU
history.