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I have a 2003 Corolla and have also dealt with the P0420 code problem.
Here's what I've discovered. I'll try to minimize the technical aspects.
Some of this you are already familiar with, but I will post it for the benefit of other readers.
The P0420 codes indicates that engine computer(ECM) has determined that the catalytic converter is not cleaning the exhaust as well as it should. This determination is done by monitoring the oxygen sensor readings (there are two of them on your exhaust system, one before and one after the converter). When the converter is working properly, the sensor readings will differ because the exhaust has been cleaned by the coverter in between the two sensors. If the readings are too similar to each other, the ECM assumes that the converter isn't cleaning the exhaust and the ECM sets off the P0420 code.
For most cars, this means the cat converter needs replacement. However, on the Corolla, that's not necessarily the case.
If you are lucky, you may get away with having your ECM reprogrammed.
Turns out that Toyota set the 'prescribed range' for the minimum difference in the sensor readings on the 2003 and 2004 Corolla very narrow. More narrow than it needed to be set to meet EPA guidelines. Thus, even though the ECM triggers the P0420 code, your emissions may still be within EPA limits. Toyota recognized this and issued a service bulletin, EG028-04 (You can google that bulletin number and find a copy of the bulletin).
The remedy in the TSB bulletin is to reprogram the ECM and put in a wider range for the oxygen sensor readings. The cost to reprogram (or 'flash' )the ECM is about $150.
Unless you are getting more MIL codes than just the P0420 code, I think its unlikely the problem is caused by a faulty oxygen sensor. Usually, a bad oxygen sensor will usually trigger it's own, separate MIL code in addition to the P0420 code.
However, since your car has over 100K, the threshold for replacement suggested by the sensor manufacturers, it probably makes sense to replace both sensors and see if that fixes the problem.
A faulty upstream sensor will usually cause the fuel mixture to become too rich resulting in high gas consumption and emissions. So if you've seen a reduction in your gas mileage the upstream sensor may be the culprit.
Replacement sensors will cost about $75-100 each plus labor to install the sensors. It's a fairly easy hour or so repair on the Corolla. I recommend getting the special socket for removing the sensor. It's a good $10-15 investment.
Replacing the Converter
For my car, the reprogramming didn't work. The car has 175,000 miles and the converter was just worn out. If your cat. converter needs replacement, the dealer will want over $1200 just for the parts. Probably close to $1500 with labor.
That was way more than I cared, or had to spend. I installed a good quality direct fit aftermarket converter which cost about $400 from an online parts supplier and carried a 5 year 50K mile warranty. I installed it myself but I estimate a garage would charge you $100 or less for labor to install it. I estimate the job should take an hour or so if you have the car up on a lift.
Doing it yourself on your back, under the car, will take much longer. The big unknown in the repair is whether the four bolts (two at the exhaust manifold and two where the pipe connects to the muffler) will release or break. I soaked mine in penetrating oil before trying to remove them. Be sure to use a six point rather than 12 point socket for better torque. It makes a world of difference on rusted bolts. My biggest problem was getting the old flange gasket off the exhaust manifold. I ended up using a cutting disk in a dremel to get it off.
I highly recommend buying the direct fit replacement kit that includes the resonator pipe as well as the catalytic converter. Also be sure to get the two replacement flange gaskets and replacement spring/bolts kits for where the assembly attaches to the exhaust manifold and your muffler. Don't try to reuse your old gaskets and bolts.
Install the entire converter/resonator assembly in one piece. Be sure to leave the joint connecting the covertor and resonator pieces loose until you have the assembly installed and connected to both the exhaust manifold and muffler. I found it helpful to attached the assembly to the hanging bracket in the center of the car before trying to reattach the front to the exhaust manifold. Once it's all in place, then tighten the u-bolt clamp around the joint holding the two pieces of the assmbly.
Even though your current resonator might be ok, getting the kit insures a better fit and make the replacement much easier. Don't go cheap with a generic converter. You'll regret it unless you plan to sell the car ASAP.
Last edited by aspen007; 12-09-2010 at 12:27 PM.