OBDII emissions question.

[simple_gifts]

A few months ago, I got the CEL code P0420, substandard catalyst according to the code reader. Factory manual indicated either O2 sensor or worn out cat. Light is on even when the car is first started. I replaced the front 02 sensor and reset the computer figuring it must be the O2 sensor since it shouldn't detect a failed catalytic convertor when the engine is first started; The CEL did not reappear for several months (enough to get thru emissions test thankfully)

Long trip to my parents, CEL comes on, same code. AAP has a new code reader which indicates O2 sensors check out but 0420 is the code.

I guess the long winded post leads to this simple question I should have just posted.

For emissions related codes, when it throws a code, does the CEL stay illuminated although the problem may have corrected itself? Odd how a failed cat can throw a code 1 second after the engine is started, or it this a code from "days ago"

Thanks.

 

[JHZR2]

Substandard catalyst could be bad cat, or could be a mismatch between the front and rear sensors. You should have replaced the rear one too. If that fails, then the cat would be replaced.

 

[Bill in Utah]

Originally Posted By: JHZR2
Substandard catalyst could be bad cat, or could be a mismatch between the front and rear sensors. You should have replaced the rear one too. If that fails, then the cat would be replaced.


+1
When I had my 0430 code (same as 0420 but other bank) I replaced only the rear one and its never come back. The pre-cat (actually all the others) O2 sensor are still OEM ones from 128k ago.

 

[simple_gifts]

I bought the rear one also. Toyota wired up the rear one to a sealed hole in the underside of the car; when I pulled the seal off, there was no quick disconnect necessitating cutting into the system. I did not have those tools readily available so I abandoned that for the time being.

It appears I will revisiting that next weekend.

Original O2 and cat; I guess I can't complain @ 250K miles.

On a side note, after my OP i went out to my car for a gas run and the CEL was off.... lol.

 

[simple_gifts]

Thanks for the input BTW; This board is such a great resource. People spend time responding and digging up old posts for people they barely know.

 

[wiswind]

A P0420 code is either a bad post catalytic converter oxygen sensor, tired catalytic converter or ECM (computer).
I have a 2003 Sienna....and that is the order that I have read that dealerships look at the problem.
I listed them in order of expense.

The downstream (post catalytic converter) oxygen sensor has 1 mission in life......monitor the catalytic converter to make sure that it is working properly.

The upstream (before the catalytic converter) sensor(s) are used by the computer to control the engine....such as how much fuel to spray in through the fuel injectors.
These sensors are VERY unlikely to cause a P0420 code.

On my vehicle, the rear (post cat) oxygen sensor leads go up through the floor of the car and plug into a connector at the base of the passenger seat......front, left corner.

I removed the trim panel that goes along the base of the seat at the carpet.......pulled back a corner of the carpet and took THIS picture of the oxygen sensor plug.



So THIS is what it looks like when you unplug it and push the grommet and plug down through the floor.

 

[simple_gifts]

Wow pictures too, thanks wiswind. My replacement has that exact grommet built in. I don't know why I never thought about servicing it from above. This is like solid gold delivered to my door.

 

[mechtech2]

Throw enough parts at it, and you'll fix it!

Codes can be stored, and not show up on the dash. And they also can come and go without being stored.
Believe me, there are no firm rules.

So what is your problem for this very common 420 code?
Assuming you have no vacuum leaks or fuel pressure anomalies, that leaves the front or possibly back sensor, or the cat.

Not much there.