P0420 comes back after CAT and O2 sensors replaced

Hi,

It is raining today, on my way home, the check light turned on and I checked, it reads P0420. I just have CAT and 2 O2 sensors(front adn back) replaced about a month, how come? what possibly went wrong? What should I do next?

Reply to
wenmang
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Reply to
mrsteveo

Reply to
wenmang

This might interest you:

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Perhaps the 02 sensor after the cat is to blame.

Reply to
Anthony Fremont

My guess would be that the replacement catalytic converter is defective or something is contaminating the catalyst.

Leaded fuel or huge amounts of engine oil leaking into the combustion chamber (with accompanying blue-gray clouds out the exhaust) could contaminate the catalyst, but if you don't see the blue-gray smoke and you haven't used leaded fuel, then a defective catalytic converter is likely.

I would bring the car back to the place that installed the cat and sensors for a warranty repair.

Good luck!

Reply to
Ray O

you:

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Reply to
wenmang

Did you read all that material? It listed quite a few things to consider.

Reply to
Anthony Fremont

A bad after-cat O2 sensor is unlikely to give a false P0420 code.

Think of the O2 sensor as those EKG sticky things and the signal from the O2 sensor and the EKG. During closed loop operation, the pre-cat sensor will put out a signal like an EKG. The cat should clean things up so that the signal from the post-cat sensor should be less lively. If the signal from the post cat sensor is the same signal as the pre-cat sensor, (lively), then the cat is dead.

Reply to
Ray O

The OP replaced the cat, pre-cat O2 sensor (referred to as sensor #1), and post-cat O2 sensor (referred to as sensor #2).

DTC P0420 will be set if the signal from sensor #2 is the same as sensor #1. In other words, sensor #2's signal should be kind of flat instead of lively. If the signal from sensor #2 is lively, then P0420 comes on. A bad O2 sensor is more likely to put out a weak signal than a lively signal so a false P0420 due to bad sensor #2 is not lively.

Reply to
Ray O

messagenews: snipped-for-privacy@k70g2000cwa.googlegroups.com...

is defective or

Reply to
wenmang

No regular gas stations sell leaded fuel, and AFAIK, leaded fuel is not available in the U.S., but I like to cover all bases when I think of it ;-)

No smoke out of the tail pipe is good!

If the place that replaced the cat does not have a scan tool, then how did you get the code? If you have a personal scan tool, perhaps the shop that replaced the can use it to check the codes. You can chcek O2 sensor operation with a volt meter.

If the cat is aftermarket, I'd bet that the replacement cat is bad.

Good luck!

Reply to
Ray O

The scan tool I mean is the equipment to scan CAT system including O2 signals not the OBDII reader. I went to the local exhaust shop this morning, they were not happy. They claimed that it was impossible for a brand new CAT going bad within a month, there must have something wrong with my car. They argued that there are a lot of factors which might cause P0420 code set by the computer and they asked me to send my car to dealer for check up, and they would not honor the warranty unless dealer says it is bad(they were not convinced that CAT could be defective). I am wondering whether there is other factor(s) that may set P0420? I like to cancel the code and wait to see what will happen next since it just passed the inspection and I have a year to find out. I am wondering whether there may be false alarm for some unknown reason. Anyway, it is very frustrated for me having to deal with P0420 code allover again.

messagenews: snipped-for-privacy@m7g2000cwm.googlegroups.com...

stations sell leaded fuel, and AFAIK, leaded fuel is not

Reply to
wenmang

I personally prefer dealership service departments to avoid the very predicament you are facing. Toyota requires dealerships to have certain equipment, among them an exhaust gas analyzer, their technicians are factory-trained, and the warranty is backed up by the most financially secure car company in the world.

The logic of how the ECM sets Diagnostic Trouble Code (DTC) P0420 is such that the most likely cause is a bad catalytic converter. #1 and #2 O2 sensors work the same way and detect the presence of oxygen (O2) in the exhaust.

Sensor #1 is sniffing the exhaust gas before it passes through the cat, and with the injectors sending fuel in pulses, the amount of O2 in the exhaust gas varies, so the voltage signal from Sensor #1 varies up and down.

Sensor #2 sniffs the exhaust gas after it has passed through the cat, and the cat is supposed to clean up the gas. If the cat is working, the the voltage signal from sensor #2 should be much more even than the signal from sensor #1. If the signal from sensor #2 is varying and looks the same as the signal from sensor #1, then the cat has not changed the exhaust and the ECM sets DTC P0420.

Other possible causes of P0420 are a damaged exhaust manifold or exhaust pipe (you would probably hear an exhaust leak), engine coolant temperature sensor not working properly, retarded spark timing, or bad O2 sensor.

At this point, your best bet is to have a dealership diagnose the condition and with a written diagnosis, go back to the shop that did the work if something they replaced is bad.

Reply to
Ray O

Thanks Ray. Dealer charges 3 times than the local shop and for my 2000 Corolla with 157K on it, it may not be worth to pay that much other than buying a new one. I will cancel the code first, it comes back again, I'll send it to dealer for diagnosis.

I personally prefer dealership service departments to avoid the very

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Reply to
wenmang

your explanation is accurate, but the very thought of going back or going to ANY dealer gives me cold shivers. in my experiennce, i have never found a dealer interested in anything but removing the dollars from the customer.. the disgusting charge for putting a car on the 'computer' is robbery. this corresponds to macing. ten bucks for a diagnosis is reasonable. their prices are not. the prices for repair and the parts are high enough to easily pay for the 'computer'. at this moment i have a dodge van with that code. i'll happily change the O2 sensors one at a time and if no joy, i'll change the cat. but no going to the dealer. the sensor cost is less that the computer test. my best to all, sammmm

Reply to
SAMMM

If you google "Corolla P0420") you will see that trouble code is not that uncommon in Corollas. In my experience, there are very few one-of-a-kind problems with Toyotas. Under the same operating conditions, either most cars will have the same problem for almost no cars will have a particular problem. My guess is that the dealership will be familiar with the condition and what it takes to resolve it.

Reply to
Ray O

Yes, there are dealer service departments that are incompetent and/or dishonest, but 97% of the ones I called on were honest and very competent. If you are in the San Francisco Bay area, New England, or Chicago's northwest suburbs, I can give you the names of reputable dealerships. While all of them usually charge a half or whole hour diagnostic fee, they all waive the fee if you have repairs performed at the dealership. IMO, that is a reasonable business practice.

A dealership's service department has much higher overhead than a comparably-sized independent shop.

For example:

A dealer must purchase factory service manuals for every model year for every model, while independents can get away with aftermarket manuals and do easy work, and if it is beyond what they have knowledge of, they say "take it to the dealer."

A dealer must purchase every special service tool for every model year for every model, while independents can say "we don't have that tool, take it to the dealer."

A dealer must send his technicians to technical training every year so that they are current on new technology, while independents do not have to.

A dealer uses genuine Toyota parts which are backed by Toyota, while independents can use the cheapest parts they can find, and when the parts are bad, they blame the failure on something else so they don't have to eat the labor or parts.

Routine maintenance is much more profitable than diagnosing and repairing problems. Stuff like transmission and differential service, brake work, exhaust work, struts and shocks, spark plugs, etc. is profitable work because the technician can do it quickly and usually does not run into problems. If you check with the independent shops and chains, most advertise this kind of work because it is profitable. What you don't see ads for are specials on diagnosing an intermittent electrical or drivability problem, because the profit margins for this work is so slim, and the independents don't even bother with this work.

This extra overhead is what makes dealerships more expensive than independents. Of course, I don't want to have to pay for that overhead any more than the next guy, so I do most work on our 3 vehicles myself ;-)

Reply to
Ray O

What would the cost have been to have the dealer just diagnose this problem, compared to what you've paid for the O2 sensors and catalytic converter?

In another forum, a person didn't want to pay a Ford dealer almost $180 to diagnose a transmission slippage problem, so he instead went to AAMCO, which took apart his transmission (maybe without his permission) and now wants almost $700 just to put it back together. A qualified, honest mechanic, no matter how expensive, is always cheaper than any of the alternatives.

Reply to
larry moe 'n curly

diagnose this

Reply to
wenmang

We're back to this, again?

P0420 Catalyst System Efficiency Below Threshold (Bank 1)

The exhaust stream that enters the CAT is good, the trouble is the exhaust stream coming out. The CAT is generally a very reliable part that should last the life of the vehicle, it certainly last longer than the one you had replaced.

Since the issue is the exhaust stream AFTER the CAT, there is little point in investigating stuff that checks the stream in front of the CAT, and since the CAT is generally very reliable, the remaining part that can give this code is the sensor that is mounted on the exhasut system AFTER the CAT, or the wiring harness that the sensor is plugged into.

I like to trust the wires, and I do trust the CAT. Therefore, my vote is that the sesnor behind the CAT is the cause of your problem. The sensor lives in an incredibly hostile environment the cooks everything that comes close.

We had this discussion a few weeks ago, and you apparently went with Ray's advice. As much as Ray has more experience with Toyotas that I will ever have, I do come from a troubleshooting background and I sleep well at night with my best-guess-estimate on what part should be replaced first.

Before you take another breath, replace the after-CAT sensor. You replaced plenty of costly parts that come ahead of where the OBD Code is telling you what the trouble is, try replacing the part that the code is telling you about.

Ray will retort that the code does not say to replace parts, this is true. But, the code is saying that what COMES OUT of the CAT is out of spec, but what comes out is okay when it went in, so the O2 Sensor is NOT the problem, nor is any of the parts that regulate fuel mixture. The ONLY problem can be the CAT itself, or the sensor that says the CAT is not working properly. (The problem could be the interconnecting wires, but I admittedly give them a bye.)

You haven't got a Bank 2 Catalyst System, so there is no point in discussing that.

Reply to
Jeff Strickland

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