Catalytic Convertor or O2 sensor?

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I've talked about this before but thought I'd see what you guys think about it now that I've got a deadline of March 31 to get it figured out. The Check Engine light is on in the '98 Accord. In the past, it's been intermittent and I could delete the code with my cheapo scanner and have it stay off long enough after a few drive cycles that I could get it to pass emissions. That is no longer the case. The code that comes up on the scanner is "Catalyst Below Threshhold - Left Bank" This is a 4cyl so there's only 1 bank. The code number for OBDII is P0420. I've searched on the internet and get conflicting advice wherever I look. Some say that I should replace the 02 sensors. Others say this generation Accord is know to have CC that fail earlier than you would expect. The car has 135K on it. The OEM sensors will be about $225. The OEM CC is $510. Either way, they aren't parts I just want to throw at the problem and hope it works. On the most basic level, the second sensor is not seeing whatever it needs to see in order to be within spec. That could be a bad downstream sensor not reading the correct differential. That could be the upstream sensor not reporting correctly. And it could be that the CC is not doing its job correctly, or some combination of the three.What are people's experiences with non-OEM O2 sensors as well as non-OEM Catalytic Converters? Do any brands seem to be better than most?
 

Beowulf

 
 
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Somewhere in the foothills...
My experience with non-OEM 02 sensors on the 80-Series is they are evil. I can't tell you how many guys (dozens) have tried to cobble non-OEM sensors then spent hours trying to figure out the wiring, then got them "working", then swapped them for OEM sensors a couple of months later.

non-OEM cats are OK.

If your up-stream O2 sensor is bad you will usually throw codes and your vehicle will not run properly. You might not see it other than lower MPG.

-Mike-
 

SteveLCetc

 
 
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I intermittently get the same code on our '99 4Runner. Mine is very sporadic. We just got back from Phoenix and it didn't code the entire trip. Is the external air inlet clear on the sensor? The ds sensor does nothing as far as I know, except throw the code. Sometimes the air sensor on the inlet can get oiled up (K&N or Amsoil air filter?) and throw a code - maybe that one, I don't remember - because it screws up the mixture and the exhaust sensors see the problem.

Keep us posted...
 
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back in the land of enchantment
our sube started that right at 100k miles. I put new oem sensors (before and after cat) and kept the light at bay for 45k miles. Now it is starting again. You're lucky, your cat is CHEAP!! It's $900+ for the Sube (but maybe that was an installed price, I don't remember).
 
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I intermittently get the same code on our '99 4Runner. Mine is very sporadic. We just got back from Phoenix and it didn't code the entire trip. Is the external air inlet clear on the sensor? The ds sensor does nothing as far as I know, except throw the code. Sometimes the air sensor on the inlet can get oiled up (K&N or Amsoil air filter?) and throw a code - maybe that one, I don't remember - because it screws up the mixture and the exhaust sensors see the problem.

Keep us posted...
It uses a MAP sensor for inlet air pressure and I do have a K&N filter in it. Maybe I'll put an OEM filter back in it. Any thoughts on flushing the throttle body to clear out the oil residue? I could take the MAP sensor out and try to clean it off. I could also run a bottle of snake oil (injector/fuel system cleaner) through it.If the downstream sensor is malfunctioning, then it could throw the code. But I'm sure the computer has other codes for O2 sensor malfunctions. I have not detected any change in performance, fuel economy or the smell of the exhaust.
 
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our sube started that right at 100k miles. I put new oem sensors (before and after cat) and kept the light at bay for 45k miles. Now it is starting again. You're lucky, your cat is CHEAP!! It's $900+ for the Sube (but maybe that was an installed price, I don't remember).
This car started doing this at about the same mileage as yours. I could erase the code and sometimes it would stay off for weeks, sometimes months. But the length of time "off" has been slowly decreasing and now it comes on almost immediately after clearing the code. I haven't been able to get all the readiness monitors set without the code coming back so that I can sneak through the emissions test and get a 2-year extension to fix the problem.
 
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The ECM compares the reading values between the upstream and downstream O2 sensors. When the readings are too similar OBDII P0420 is thrown. I only happen to know this because the Lotus has thrown it once or twice after coming off the turbo at high RPMs. I think the dramatic change in exhaust character temporarily overwhelms the CAT the code is thrown. Marc, in addition to the O2 sensors and CAT you might also check your coolant temp sensor for correct readings and also verify there are no upstream exhaust leaks because will also lead to that error code. I have also "heard" that an engine burning oil can temporarily foul the CAT.
 
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I would assume upstream exhaust leaks would be easy to find by the sound. There aren't any obvious leaks that I'm aware of. I'm very reluctant to just replace the CAT because of its price. At the same time, I don't want to just throw a $115 O2 sensor (or two) at the car and hope that fixes it. I have not looked at the Honda tech site to see if there is a bench-test procedure for determining of the O2 sensor are functioning properly but I'm not sure what you could do outside of an engine exhaust stream to get the same temperatures to test it. The only test I could conceive would be checking the resistance at cold and normal operating temp.

Burning oil - The car does consume about 1/2 quart every ~3K miles. I don't see smoke and I can't find any leaks. I would like to think that is a slow rate of consumption and would not foul the CAT, but I don't really know.
 
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It uses a MAP sensor for inlet air pressure and I do have a K&N filter in it. Maybe I'll put an OEM filter back in it. Any thoughts on flushing the throttle body to clear out the oil residue? I could take the MAP sensor out and try to clean it off. I could also run a bottle of snake oil (injector/fuel system cleaner) through it.If the downstream sensor is malfunctioning, then it could throw the code. But I'm sure the computer has other codes for O2 sensor malfunctions. I have not detected any change in performance, fuel economy or the smell of the exhaust.
Everything in the induction is pre-combustion (obviously) so it shouldn't have anything to do with OBDII P0420. That said, make sure the engine is in proper tune because that will affect the exhaust character.

I also have another "theory" that the gasohol used in Bernco from November to February may influence the CAT efficiency. Why not try a tank or two of good quality gas from a Sandoval County station?
 
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It got new OEM dist. cap, rotor, plugs and wires when it got a new timing belt about 20K miles ago. It's in proper "tune". I might try the Sandoval County gas idea. How far north do you think I might need to go to find a pump that doesn't have the added ethanol? I know there's a sticker on the pump warning of the addition of it in Bernco
 

alia176

 
 
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In my 80, when I got the same error code, I had an exhaust leak between the two sensors, before the cats. After the leak was repaired, the fault code went away. Both sensors are identical so you can switch them around to see if that does anything, but it prolly wont! Curious as to how you end up solving this issue.
 
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According to the Honda parts system, my two sensors are not the same. They have different part numbers and they are about $15 difference in price. As of last night, all the readiness monitors were set and registered ok after erasing the code on Saturday. I told Danise to get herself to the emissions testing station ASAP, before the fault has a chance to return. I also filled up with 91 octane as well as added a bottle of fuel system cleaner last night. I'm not sure the premium fuel or cleaner will add any value to the equation but figured I'd give it a shot.
 

SteveLCetc

 
 
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Consider it your baseline anyway... One popular method if using white gas, which would be the opposite approach you are taking.
 
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PASSED!!!

Danise took it in to the local testing station and it passed. I get a 2-year postponement of the headache.

The technician did something funny though. After plugging into the OBDII port and then testing the gas cap, he proceeded to actually use the exhaust gas analyzer, putting the tailpipe probe into the exhaust stream. I have never had any technician actually test the exhaust on my OBDII car. The good news was that he made a comment to Danise that "the exhaust numbers are fantastic, considering how old the car is." He did not quantify "fantastic" and there weren't any numbers printed on the ticket like there usually are on my older vehicles that must have the exhaust gasses tested.

If the actual emissions numbers are "fantastic", that tells me that the cat is not the actual problem and it is a sensing malfunction that is turning on the indicator.
 
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