P0420 CEL Check Engine Light Question

[biggles99]

I have a 2000 Corolla with 181K on it.

Here is the lineage of my issue:

1. Have had a CEL on for a while now (3 months). I know, not good I left it so long, but we continue...
2. Also had a rattle coming from under the car, seemingly exhaust related, kind of a rattling sheet metal sound.

Tackled #2 first.

2a. Got under the car, and noticed that one of the bolts on the top portion of the 90° bend of the exhaust, before the catalytic converter is, was gone.
The shiny zinc plated bolt is the one I put on about 5 days ago.




2b. With that bolt being gone, this thing was loose and had slid down somewhat (1/2" or so?), was this thing:



Not sure what that is, but it is presumably inside the catalytic converter doing something.

It is also just about exactly where the "bank 1" oxygen sensor is. The sensor before the catalytic converter (but on the opposite side of the exhaust tube):



I think that my new bolt is of a slightly smaller diameter than the original one, so I am not sure it is in the same exact position or not. Didn't think it mattered a whole not. Not so sure now.

(going to continue on next email so I don't lose what I have so far...)

[biggles99]

2c. Reset the computer by disconnecting the Neg battery cable.
2d. Rattle went away and has not come back, so that is that.

Back to #1:

1a. After a day of driving, CEL came back on. Went to get code read. Got both P0171 and P0300. After reading, decided to clean the MAF sensor. Reset CEL code again.
1b. Drove 20 miles to work, no light
1c. About 15 miles back, CEL again
1d. Did not check specific codes, but changed spark plugs with NGK Laser Iridiums. The existing ones ended up looking pretty bad.
1e. Drove 20 miles to work, no light
1f. About 15 miles back, CEL again
1g. This time, checked the codes again. This time, P0420 showed up twice, same code. Most things I read make it related to the two oxygen sensors telling you the cat is bad, or not working.

That was yesterday.

The more I read, the more it is pointing me to the catalytic converter being bad. However, with that one bolt being gone, not sure if that thing has anything to do with how the first oxygen sensor does its thing though, and maybe trashed that sensor.

One main problem: By the first week of Nov, I need to pass an e-check (Ohio), and I am worried I will have to change out the whole catalytic converter, which I did not want to put into a car that I hope to only have 6 more months or so (or less).

Questions, for anyone nice enough to have read though all of that:

a. I am thinking of changing out the first oxygen sensor first. Does that seem logical? To a degree, I don't think that it does, because as I read it, if it was the oxygen sensor(s), they start doing their checking very soon after the car warms up. I would think I could not go 20 miles with the CEL off if it was not working. I figure it would turn the CEL on before that.
b. Anyone know what the inside of the cat looks like that can describe what that thing was that was loose when the bolt was gone?
c. Any reason to think the P0420 code is related to anything else?

Thanks ahead of time, these forums are very good.

[wylee]

I doubt it is the sensor before the cat. I would think either the cat or the sensor after the cat. Depending on how thorough the shop is with a visual inspection, you might get away with moving the downstream O2 sensor out of the exhaust stream with a spark plug defouler to fool the emissions system into thinking the cat is working.

Do the defouler trick at you own risk...I suspect tampering with emissions equipment is frowned upon by johnny law.

FWIW...I just bought a cat and downstream O2 sensor on Amazon to solve my P0420 problem. Maybe I didn't need to do both, but I only had a few days to get my sticker and I didn't want to screw around with it.

[biggles99]

Thanks wylee,

Any reason to think that just doing the downstream sensor by itself could be worthwhile?

Thanks

[teppista881]

If the cat is functioning properly and the O2 has sloppy readings, then changing just the sensor will definitely fix the problem. If you're going to put money into it, start with the cheaper part first.

[wylee]

Originally Posted by biggles99

Thanks wylee,

Any reason to think that just doing the downstream sensor by itself could be worthwhile?

Thanks

It is possible just replacing the downstream sensor could take care of it, but with 180K your cat is probably near the end of it's life.

In my case, my car was burning oil and had a bad coil replaced due to a misfire so I was virtually certain the cat was shot with only 130k. Unburned fuel and carbon from burning oil are both very unfriendly to a cat.

[biggles99]

Thanks everyone so far,

I am leaning towards doing the post-converter O2 sensor, but I also have a appt with the Toyota dealer to discuss as well

Thanks,

I will update as I go...thanks everyone

[jbod]

i've had those codes on my 99 corolla for a while now. maybe even a couple of years now. it was ridiculous what it cost to fix it. i used a buddies scanner, and have just let it go the last couple of years. haven't really noticed any difference in the way it runs or performs though. if anything i might be getting maybe one or two miles per gallon less. thats it.

[sam333]

The bolt that fell out, just holds on your heat insulator and has no effect on your cat or o2 sensor.

Your PO420 code is common even if there is no problem with the cat.The problem could just be a bad o2 sensor( either one). The ECM compares the forward o2 sensor oxygen levels with the rear o2 sensor levels. In a properly working cat the o2 levels will be lower at the rear o2 sensor. If they are the same the ECM sees the cat as bad and gives you a 420. The ECM uses the front o2 sensor to determine the proper fuel to air ratio for the engine , the rear does nothing other than tell you if you have a potential problem with the cat.

The o2 sensors are easy to test, so dont waste your money on a new one till you test them.

If you have to buy a new one buy a Denso only, to many problems with the other brands.

Your PO300 could be caused by a vacuum leak which could also cause your PO420.

On a car with this much milages ,I would bet your front gasket( doughnut), the one between the exhast manifold and front exhast pipe is bad/ leaking. This can cause problems with the o2 sensors by pulling in air . A telltale sign is if you drive in the rain( over wet ground / puddles) , you will hear the exhast sound change( gets louder).

[boxer4luvr]

Originally Posted by sam333

The bolt that fell out, just holds on your heat insulator and has no effect on your cat or o2 sensor.

Your PO420 code is common even if there is no problem with the cat.The problem could just be a bad o2 sensor( either one). The ECM compares the forward o2 sensor oxygen levels with the rear o2 sensor levels. In a properly working cat the o2 levels will be lower at the rear o2 sensor. If they are the same the ECM sees the cat as bad and gives you a 420. The ECM uses the front o2 sensor to determine the proper fuel to air ratio for the engine , the rear does nothing other than tell you if you have a potential problem with the cat.

The o2 sensors are easy to test, so dont waste your money on a new one till you test them.

If you have to buy a new one buy a Denso only, to many problems with the other brands.

Your PO300 could be caused by a vacuum leak which could also cause your PO420.

On a car with this much milages ,I would bet your front gasket( doughnut), the one between the exhast manifold and front exhast pipe is bad/ leaking. This can cause problems with the o2 sensors by pulling in air . A telltale sign is if you drive in the rain( over wet ground / puddles) , you will hear the exhast sound change( gets louder).

That's EXCELLENT information Sam!!