Catalytic converter



Recently my wife was driving our 98Toyota Corolla when the chechk engine light came on. We took it to our mechanic who hooked up the sensors and said bad Catalytic converter. We then took it to our local Toyota dealer and he said bad catalytic converter and there was a good muffler shop down the street who could do it for about a fraction of the $1200 bucks they would charge. I know it may be something else like an oxygen sensor or the muffler. My crazy 83 year old uncle says forget it and don’t have it replaced. I am an old worry wart and don’t want my wife stuck on the highway. Should we get it replaced, get an emission check to help narrow it down, or listen to my crazy old uncle?


What was the exact code that the technicians got? Should in the the form P####.

Chances are, if there’s nothing wrong with the way the car drives and behaves, I’d say it’s okay to leave it for now. But post up the code.


I doubt if it will leave you stranded, but we need the code.

That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code not just their translation into English and post it back here. It likely will have a format like P1234.

Leaving it as it is would not be a good idea. With that light on or with it covered with black tape, you will never know if it gets a new code and that new code could be one indicating a dangerous situation.


The bad converter, if it is bad, will not effect the operation of your car. If your state requires an emissions test, your car will fail simply because the light is on. You can wait until then if you like. You can also make the light go off by disconnecting the battery for a few seconds see if it stays off.


Thanks for your help. I will try to get the Pcode for you from my mechanic or Toyota. If not I will contact Auto Zone.


In Ontario, a car can pass the emissions test with the CEL on. My 99 Sonata passed fine with the CEL on, the technician said he’d test the car anyway and see if it passed. It did, and the CEL is still on. I don’t know why, and I don’t really care. I’ve got enough work cut out for me keeping my 97 Crown Vic is top condition.


Emissions testing is a joke no matter what form it takes. Not enough cars fail these “tests” to make ANY measurable difference in air quality. They amount to nothing more than a tax on motorists. The only thing accomplished is money changes hands. Same deal goes for “Safety Inspections”…If someone wants a sticker bad enough they will get one…


You can verify some things with other tests. If a smog check indicates 250 ppm of HC, that would really prove, in addition to the last diagnosis, that the converter is really bad. But if the cat is bad due to blockage, a backpressure test would show it. What test was done and what were the results?


Amen to that, Caddyman.


Thanks for looking at my query. I called toyota this morning and they said it was a code PO 420. Thanks for your help.


The only thing they did was hook up the sensors and get a code PO 420.


and there was a good muffler shop down the street who could do it for about a fraction of the $1200 bucks they would charge.

The dealership uses a factory replacement cat. while the muffler place uses an aftermarket one, wich is just as good and can be picked up for something like $150~300 depending on the car.


That’s where you are wrong.

If the convertor material breaks apart or melts down it could plug the exhaust and that will definitely affect engine performance.

I agree that emmission testing is a joke.


The OP has no performance complaints…Usually, when the CEL comes on and flags the converter, it’s because the o2 sensors (front and back) have the same readings, indicating an inactive converter…This code can sometimes be generated by a bad O2 sensor giving a false reading and confusing the computer…The parts changers will of course replace the converter AND the oxygen sensors just to cover all the bases…


I have an '03 Mazda MPV with 100k. CEL came on steady. Dealer charged $95 for diagnostic and told me I needed a new converter. I think they just read the code (P0431)(last O2 sensor on 2nd converter) and did not check for (or at least have any record of) an exhaust leak test, nor the O2 sensor voltage, nor temp. of gas going in and out of the converter, nor the back pressure, nor the efficiency of the converter - they just took my $95 and told me the “master technician” thought I needed a new converter for $1,100 (which I declined). If you are asked to pay for $100 for a diagnostic check for a CEL, be sure to ask for detailed results up front. My MPV had just passed the MA inspection 2 weeks prior and runs great with no unusual noises. They cleared the code and so far after about 10 local and LD trips and 100 miles the light has not come back on.


I agree with you in this case.

I take issue with the statement that a defective convertor will not affect performance when in fact it can if it plugs the exhaust system.


What a RACKET this stuff is…A little light comes on and people fork over big bucks out of total ignorance about their cars…

Every car should have a little LC screen that displayed any “trouble code” in plain English. That would cost Detroit about $10 to install. Saving motorists millions and millions of dollars in dubious “diagnostic fees”. This stuff borders on organized crime…


I don’t think a downstream O2 sensor can cause a P0420 DTC. If the downstream O2 sensor fails, it remains high (900 mV) or low (100 mV or 0 mV) and does not repeatedly cross a mid-level reference point of about 450 mV. The ECU continuously monitors all sensors, and if it notes that one of the O2 sensors has stopped fluctuating, it sets one or more other DTCs, but not the P0420.

In a good cat, the downstream O2 sensor lazily follows the upstream one. One method by which the ECU sets the P0420 DTC is by counting and comparing the number of 450 mV threshold crossings in the upstream and downstream sensors over a period of time. If the downstream crossings are, say, 70% or more of the upstream count, the cat is considered defective. (In the extreme case of where the cat were doing no job at all, the downstream O2 sensor would mirror the upstream O2 sensor and the counts would be identical.) The OS2 sensors can be easily tested on the car with a DSO (Digital Storage Oscilloscope), which most shops have nowadays.


Even if they put it in, that doesn’t mean most people will truly understand it, no matter how simple the “language” was.

Pay no attention to the white, windowless van that is currently parking across the street. shifty eyes