catalytic converter and oxygen sensor replacement?

I mean check light was on

When the check engine light comes on, the car's computer usually stores a trouble code. Code P0420 means that the catalyst sytem's efficiency for bank 1 is below the proper threshold. The Corolla has an in-line 4 cylinder engine so there is no bank 2 (unlike an engbine with a "V" or horizontally opposed configuration). Basically it means that the system thinks that the catalytic converter is not working efficiently. The catalyst in a catlytic conver does not wear out but it can become coated with contaminants over time so that the catalyst can't work on the gasses passing over it.

The catlytic converter and oxygen sensor do not necessarily have to be replaced together. An O2 sensor that is marginal could give the same trouble code so the shop is probably covering all possibilities as to the source of the trouble code.

If good quality gas is used, a catlytic converter should theoretically last the life of the car. An O2 sensor's life can be anywhere from 60,000 miles to the life of the car.

I believe that cheap gas can shorten the life of the O2 sensors and catlytic converter, which is why I try to stick to name brand gas. It helps to get the exhaust system good an hot every so often by taking it out on the highway in 3rd gear for a few miles.

The engine is attached to the chassis in 3 places with a rubber block. The rubber block allows the engine to flex a little and dampens the engine vibrations that are transmitted to the passenger compartment. The rubber in the mount sometimes becomes hardened or cracked. This is not a common problem in Corollas and I would not worry about it unless you get strange noises when accelerating or shifting from acceleration to braking.

Yes.

Thanks a lot, Ray.

What is "sytem's efficiency for bank 1"? I was asked by an exhuast repair shop for upstream or downstream O2 sensor, which one they were asking about? I may have them replaced by an exhaust repair shop like Midas instead of dealer. The other thing is that I am not that handy, I can do some simple maintainance work, like oil change, front brake replacement, but how difficult to replace them by myself using after market parts?

Thanks.

I was also told that my car had two O2 sensor, according to what you just said, was he mistakenly stating extra O2 sensor? Ny car is 2000 Corrola VE with 4dr Thanks.

Ray, Can you discuss for the OP, the Upstream and Downstream O2 Sensors, and whether his car even has them? I don't know if he has the down stream sensor, and my inclination is to replace the O2 Sensor that is on the exhaust manifold, which is the upstream sensor.

I agree completely with your advice here, but I'm a bit confused by the particular code he got. My gut instinct is that this would be a downstream O2 Sensor, but I don't know that his car has this or not. Since he has the code, I have to assume he has the parts. I was going to tell him to replace the O2 Sensor(s) because, like you, I think the CAT should be pretty much a lifetime item in most cases.

This is one of those times when my shadetree expertise just doesn't cover what's going on.

The O2 Sensors are parts that you can replace yourself, so to answer your question, yes, you can take the car to a muffler shop instead of to the dealer.

The O2 Sensors are generally screwed in place in much the same manner as a spark plug. There is a special socket wrench (cost is under $10.00) that can accomodate the wire. The special wrench looks much the same as a spark plug socket, but has a slot cut in the side for the wire. Simply disconnect the wire at the connector, and unscrew the old sensor and put the new one in.

I get that you might not be much of a mechanic, but my point is to express how basic and simple this job is. It's like taking a spark plug out and putting in a new one. There can be issues of corrosion and heat damage that can make the job a pain in the ass, but if it goes right, it's really very simple.

A catyltic converter ("cat") cleans up the exhaust gases. If it is not doing its job because the catalyst is contaminated or coated, the second or "downstream" O2 sensor will detect this and the computer will turn on the check engine light.

The Corolla has a 4 cylinder engine, with all 4 cylinders lined up in a row, referred to as a "bank." This layout is known as an in-line engine. Other engines have cylinders arranged in a "V" or "H" configuration. Think of a V-6 or V-8 engine as two 3 or 4 cylinder in-line engines joined at the bottom in a "V" shape. These engines have 2 rows, or banks of cylinders.

I was asked by an exhuast

Your car has two O2 sensors. Sensor #1 (sometimes referred to as the "upstream" sensor) measures the oxygen in the exhaust as it exits the engine. The car's electronic control unit ("ECU" or computer) adjusts the amount of fuel injected into the engine based on what O2 sensor #1 sniffs in the exhaust.

After the exhaust gas passes through the cat, O2 sensor #2 (sometimes referred to as the "downstream" sensor) takes another sniff and sends a signal to the ECU. If the exhaust gas is not clean enough, the ECU assumes that the cat is not working properly and turns on the check engine light. It is possible that sensor #2 is not working properly and thinks it is smelling too much pollutants in the exhaust.

One possibility is that whatever contaminated the catalyst could also contaminate the O2 sensor, which is why you would need to replace both at the same time.

Personally, I wouldn't bother with the O2 sensors unless you get another check engine light after the cat is replaced and the ECU points to one or both of the sensors.

If you can change the brakes on your own car, you can change the O2 sensors. Sensor #1 is probably located in the exhaust manifold, visible when you open the hood. It will be screwed into the manifold with a wire connector coming out of it and is about the diameter of your finger. Sensor #2 looks just like Sensor #1 and is located in the exhaust pipe after the cat.

I described the "upstream" and "downstream" O2 sensors in my reply to the OP. I don't like the terms "upstream" and "downstream" because you will not find those terms used in a repair manual, just like I cringe when people call wheels "rims." Look in an auto repair manual and see if there is a procedure for mounting rims or balancing rims.

I believe that most, if not all, OBD II vehicles have 2 O2 sensors. O2 Sensor #1 provides feedback for fuel mixture and sensor #2 measures catylitic converter efficiency.

The cat is lifetime in theory. In realty, the catalyst gets coated with contaminants like soot or residue from burning oil, or contaminants in the fuel. Taking the car on a highway for a good blowout every so often gets the cat good and hot and helps clear the pipes, so to speak.

An O2 sensor has a much smaller surface area than the catalyst bed so it probably doesn't take as much to clog it's "nose."

Your car does have two O2 sensors, one in front of the catalytic converter, close to the engine (possibly in the exhaust manifold, or just after), and one after the catalytic converter. The one before the converter is used by the engine computer to adjust the fuel mixture. The one after the catalytic converter is used to make sure the converter is working.

Personally, I'd try replacing the O2 sensor after the converter before I bought a catalytic converter. You really should not need both. If the converter failed, the O2 sensor after the converter is probably OK, since it warned you that the converter had failed. If the converter is still OK, then the sensor after the converter must be bad since it identified the converter as bad when it wasn't. No reason for both to fail at the same time.

You said you recently passed your states inspection. Did they actually measure exhaust emission during their test? If so and you passed, I'd be inclined to think the catalytic converter was OK.

Ed

What if "sensor #2 is not working properly and thinks it is smelling too much pollutants in the exhaust."? Is there a way to check #2 O2 sensor good or bad first before having CAT replaced? CAT replacement is so expensive, I hope that I can identify the condition of #2 O2 sensor.

Thanks. WM

If I replace #2 O2 sensor before replacing CAT, is it possible that the new O2 sensor get damaged by sniffing too much the "dirty stuff" because of a bad CAT?

Believe or not, the exact same dealer did the state inspection on the exact the same car, it passed PA inspection.

Possible but highly unlikely in a short period of time (under a mnth).

To check O2 sensor operation, you will need a volt/ohm meter or oscilloscope.

Start the engine and let it warm up to operating temperature, turn accessories off. Run the engine to at least 2,500 RPM to heat up the O2 sensor. You should get a reading that swings from less than 400 millivolts DC to greater than 550 millivolts DC at least 8 times in 10 seconds. If the lower voltage reading is above 400 mv or the higher voltage reading is not at least 550 mv then the O2 sensor is bad. If it doesn't swing back and forth

And Bank 1 or Bank 2 would refer to a dual exhaust system, where I assume Bank 1 would be the left side, and Bank 2 the right? If the two banks came together to a single exhaust from the CAT back, then there would not be a Bank 2 "downstream" sensor. Sensor 1 is the upstream sensor, and Sensor 2 is the downstream, where up and down are before or after the CAT.

Sensor 1 plays a role in manipulating the fuel mixture to achieve a particular result, and Sensor 2 merely reports if the CAT is working. I think I have that right.

My limited experience with O2 Sensors is that they simply cook. They live in a VERY hostile environment, and cooking is a very real issue for them. Especially the upstream sensors, I had one that crumbled and fell apart.

Given that the CAT is a lifetime part, then it makes sense to replace the sensors before buying a new CAT. Sure, the CAT can go bad given enough time, or a bad load of gas, but that really isn't something that happens with any regularity until the car becomes advanced in years. I had an '89 Ford chassis motorhome, and the CAT is fine after more than 15 years, so my guess is that the OP's CAT is still serviceable.

That is most likely what is going on. The sensor is failing -- or has failed -- and is sending a false report. There's a chance that the CAT is toast, but if I was doing the job on my own car, I'd replace the sensor before I did anything.

I'd actually reset the Check light, then wait to see if it comes on again. If it does, AND it has the same code P0420, then I'd replace the sensor, reset the code, and wait again.

I'm with Ray on this one, replace the sensor. If the CAT is bad, it won't hurt the new sensor right away. My money is on the idea that the CAT is fine, the sensor is giving a false report.

Not quite. Bank 1 and Bank 2 refer to cylinder banks in a V or horizontally opposed engine configuration.

If the two banks came

I never thought about it but this is probably correct.

Sensor 1 is the upstream sensor, and Sensor 2 is

Correct.

Correct.

I haven't run accross that myself. On Toyotas, they will usually throw a check engine light before they start to crumble.

Since it is possible to check an O2 sensor's function, I would check it before replacing it, and yes, I would check it before blindly replacing a cat unless the cat is still under the emissions warranty, where you get to start over with a brand new $1,500 part.

Cats go bad for a number of reasons. The catalyst can get coated with soot or some other contaminant so that the exhaust gas doesn't actually come in contact with the catalyst; a hole can develop in the case; the structure that holds the catalyst deteriorates and collapses, clogging the exhaust; or debris from the exhaust ahead of the catalyst can clog the cat.

do all cars have the sensor#2 after the CAT?

I had a problem with my wife's Nissan Pathfinder and I thought I saw that bank 1 & bank 2 had sensor #2 right before the CAT.

Sorry, my bad. I was thinking of a motor where the fan was in the front and the bellhousing was on the back, not the left and right, respectively or vice versa. But, yes, Bank 1 is the exhaust manifold on one side, and Bank 2 is the manifold on the other, where "side" is relative to the orientation of the crankshaft.

I'm sure my Jeep really wanted to throw a code ...

I agree with all of that. I think the odds of a failed CAT are much lower than the odds of a failed sensor, and the sensor is cheap and easy. Observing my Cheap & Easy Rule where the cheap and easy parts are replaced ahead of expensive and difficult parts, I'd replace the sensor first.

I like your test, but don't have an oscope, so I have to replace stuff in accordance with my Cheap & Easy Rule. I realize this isn't always reliable, but it works more often than not.