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1999 Toyota corolla- catalytic converter error code


I have a 99 toyota corolla with 147,000 miles on it. The Check engine light of my car keeps coming on intermittently. I got the code checked and it says catalytic converter below threshold. Usually the light goes away by itself in a day or two, however that didn’t happen the last time it came on,which was 6 weeks back. I took my car to the dealership hoping they would tell me exactly what’s wrong i.e. whether it’s the sensor (front or back) or it’s the converter. Well, that didn’t happen and the dealership advised that I get the exhaust pipe, catalytic converter and both the sensors replaced for a price tag of $2,500. At the time they erased the code and the light was gone, however it’s back now and I am looking for a good solution. My car is in a good running condition…gave me about 38 mpg in summer, unfortunately it’s down to 33 mpg right now and I am not sure if it’s the cold whether that’s responsible for lower mileage.

Please advise on how to find out if the sensor is bad or if it’s the converter. Also, is it normal for corolla to give lower MPG in winter?

Thanks for your advise.


My advice is to stop using the dealer for repairs since they are no longer covered under warranty.

  Dealers are no better (or worse) than independent mechanics for almost anything you might need done on your car.  They will almost always charge more per hour and often more for parts and supplies.  They also tend to look at repairs a little different than the independent. 

A dealer may well recommend work that strictly may not be needed, but could be connected to the problem or maybe replace a part when a little repair would fix it ALMOST as good a new.  

There is no need to bring your car to the dealer for any service other than service that is going to be paid for by a recall or original warrantee.  During the warranty period be sure to have all required (as listed in the owner's manual) maintenance done and to document all maintenance work.

I suggest that most people would be better off finding a good independent (Not working for a chain) mechanic. 

Note: Never ever use a quick oil change place. They are fast cheap and very very bad.

Find a reputable independently owned and operated shop that can read a scope. The signals from the pre and post catalytic coverters can actually be looked at and a deterination can be made whether the sensors are operating properly and indicating a bad converter. Realize, however, that this is not a perfect science. Beyond looking at the traces, there really is no way to definitively tell absolutely for certain that the converter is or is not bad. A bad sensor can give a deceptive trace.

Now, about the cost…direct fit (OEM replacement…NOT OEM) parts can be bought and installed for a fraction of that quoted price. You have a single, straightforward exhaust system with a single muffler serving a modest 4-banger. Get some prices from part stores and you may be pleasently surprized and decide to just go ahead and replace the whole thing.

Oh, the difference between OEM replacement and OEM parts is that the “replacement” parts will be from an aftermarket manufacturer that may or may not not sell directly to the car manufacturer. An OEM will have a Toyota part number but may be exactly the same parts. It has to do with “configuration controls”, “spec control drawings”, and othe junk that while important to the manufacturers has no effect on you…other than price.

Bottom line – stay away from the dealer. You didn’t have both the CAT and sensors go bad at the same time. Most shops can read your sensors with a scan tool to see how they’re performing. A simple digital thermometer with a laser light that points at the CAT will tell if it is working or not. The hand held model that you shine the laser at and it gives you a temp. can be bought at Harbor Freight for 49 bucks. The back of the converter should be hotter than the front of the converter. If that is the case, it is burning (working). If they are about the same temp, it is old and used up. Make sure the car is good and warmed up first. At 149K miles, it might be used up. The material inside doesn’t have an unlimited lifetime, it will wear out eventually. I had a CAT on my Sentra several years ago that quit working at about 150K miles but never gave me an indication because it was a carburated car, and it burned so clean it always passed emissions test, even at the 300K mile mark when I sold it. The Autozones here in Utah use a scan tool that will read dynamic O2 sensor voltages. Basically you’re looking for a lot of activity (voltages fluxing up and down from .100 - .900 V) with the sensor(s) prior to the CAT and not much activity (minimal voltage changes) post CAT. They can also do an O2 sensor readiness test, and the best part is that it is all free at autozone, schuks, etc. The hard part is finding a counter person that knows how to read the scan tool well and knows what everything means. Don’t spend the kind of money the dealer wants, I’m betting if you were to get the same parts replaced in a muffler shop, it would be 1/4 to 1/3 as much. Good luck.

Thanks for the replies…

I have been to Autozone in past and the only code they have detected is P0420 which is a blanket code for the entire system. I stay away from dealers but I was hoping the dealer would scan the system for Sensor fault or converter fault and tell me which part needs replacement. Unfortunately, they didn’t tell me anything besides what Autozone had told me and charged me 100 bucks for it.

What I am in search for is a good recommendation on how to address this issue without getting ripped off.

Thanks again for all your replies.


Technical DescriptionCatalyst System Efficiency Below Threshold (Bank 1)

What does that mean?The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

SymptomsYou will likely not notice any drivability problems, although there may be symptoms.

CausesA code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings
Possible SolutionsSome suggested steps for troubleshooting a P0420 code include:

Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2), replace if necessary
Replace the catalytic converter
One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it’s out of it’s bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It’s worth checking into.

Bennyandthejets gave you the indications one looks at when one uses a better scan tool. You may have to pay an independent shop to do this; then, you can deside what repair actions to do.

You’ve got to remember that the oxygen sensors, and the catalytic converter, are at the end of everything that has gone on in the engine. Several things shorten the life of the oxygen sensors and catalytic converter: a rich-running, or misfiring engine; burning anti-freeze (internal coolant leak); burning oil, or silicone (from sealants).

As a preventative (or, possible curative): keep the engine in turn; perform the routine maintenance; treat any misfire conditions. These actions, and similar actions, may fix your current problems.