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Discussion Starter · #1 ·
Hello all. My check engine light came on then came off and then came back on again within a span of 2-3 weeks. Finally used a scanner to check for a code and got P0420 which deals with the cat. My problem is that the car is straight piped (and has passed smog in May) so I really dont know what to do or what's causing the check engine light. Should I just replace the o2 censor? Or is there something else wrong? Thanks.
2005 TSX 160,334 miles btw.
 

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The most likely explanation of why you are getting that code is because you are straight-piped. Your car measures the emissions before and after your cat and those two readings should not be the same (as happens when you are straight-piped). If you replaced one or both sensors you'll likely get the same code until you put a new cat in there. As you can now see, the smog test and how your sensors measure are very different and thus can not be equated (one measures total output, the other measure the difference between 2 sensors).

If you don't want to add the oem cat back you can always try a high-flow cat for more performance. Ultimately if won't affect your cars ability to function except that the poor guy behind you that has to breathe your exhaust.
 

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Discussion Starter · #3 ·
The most likely explanation of why you are getting that code is because you are straight-piped. Your car measures the emissions before and after your cat and those two readings should not be the same (as happens when you are straight-piped). If you replaced one or both sensors you'll likely get the same code until you put a new cat in there. As you can now see, the smog test and how your sensors measure are very different and thus can not be equated (one measures total output, the other measure the difference between 2 sensors).

If you don't want to add the oem cat back you can always try a high-flow cat for more performance. Ultimately if won't affect your cars ability to function except that the poor guy behind you that has to breathe your exhaust.
Okay thats what I was thinking. The only thing is that the car was straight piped when it passed smog, why would the check engine light barely come on now (4 months later)?
Apologies in advance if the question can be answered with the info you've already given.
 

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I see why you are asking but I think that there is still some confusion. The smog test is 'different' from the error code. You may be able to pass the smog test whether you get the code or not, theoretically speaking. Of course if you did have the code they won't pass you.

As to why you now get the code and not before, well there may be several reasons. You may now have a bad sensor, bad sensor wiring, exhaust leak, etc. Or maybe you need a tune-up. Either way your readings are now out of the acceptable range and thus your car generates the error code. A bad sensor or sensor wiring will not affect your car's ability to emit exhaust and still allow you pass the smog test. Remember it measures output, not whether you are getting a code or how your sensors work. The smog test and how your car (sensors) generate your error code are independent and different tests.

So you may need a new cat or new sensor(s) or a combination of both. At least now you know how your sensors work so if you replace them and you still get the error code it's likely that you need a cat. Hope this helps.
 

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Hello all. My check engine light came on then came off and then came back on again within a span of 2-3 weeks. Finally used a scanner to check for a code and got P0420 which deals with the cat. My problem is that the car is straight piped (and has passed smog in May) so I really dont know what to do or what's causing the check engine light. Should I just replace the o2 censor? Or is there something else wrong? Thanks.
2005 TSX 160,334 miles btw.

How long has the car been running with no CEL and no catalytic converter?

You must have an O2 sensor extender (O2 sensor CEL eliminator) installed for the secondary O2 sensor?

How long does it take for the CEL and P0420 to be set after you clear the DTC's?

Any other DTC's other than the P0420?
 

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Discussion Starter · #6 ·
How long has the car been running with no CEL and no catalytic converter?

You must have an O2 sensor extender (O2 sensor CEL eliminator) installed for the secondary O2 sensor?

How long does it take for the CEL and P0420 to be set after you clear the DTC's?

Any other DTC's other than the P0420?
I believe since May 2021 or even before. I recently bought the car used, beginning of August to be exact.

I don't know how to answer your second question. I guess I can check for that and then come back and reply.

I haven't cleared the codes yet so I don't know. Been extremely busy with other stuff.

The other codes I got were: 61-1, 83-1, 112-1.
 

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I had a feeling there would be more to the story... :)


So you have only owned the car for about a month and the CEL has been on for most of that time?

Do you have any proof that the car passed a smog inspection in May 2021?
Whatever the previous owner told you is questionable unless you have written proof to back it up.

You have numerous other DTC's in addition to the P0420?

I highly doubt the car passed a smog inspection without a catalytic converter.
Even if it has something that looks like a cat in the exhaust.


1st up to record and analyze all the DTC's the car currently has.
Clear all the DTC's and then see which one's come back and how long and how many miles it takes.

Jack up the car and inspect the exhaust system, cat, O2 sensors and O2 CEL/MIL eliminators.





 
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Discussion Starter · #8 ·
I had a feeling there would be more to the story... :)


So you have only owned the car for about a month and the CEL has been on for most of that time?

Do you have any proof that the car passed a smog inspection in May 2021?
Whatever the previous owner told you is questionable unless you have written proof to back it up.

You have numerous other DTC's in addition to the P0420?

I highly doubt the car passed a smog inspection without a catalytic converter.
Even if it has something that looks like a cat in the exhaust.


1st up to record and analyze all the DTC's the car currently has.
Clear all the DTC's and then see which one's come back and how long and how many miles it takes.

Jack up the car and inspect the exhaust system, cat, O2 sensors and O2 CEL/MIL eliminators.





I bought the car Aug 4 to be exact, and the first check engine light came on on Aug 25 but went away the next day so I didn't think anything of it. The light came back on Sep 8 and has been on since. I really only drive the car on Mon and Wed to commute if that helps. Also located in California and car was smog here as well. Everything is registered through the DMV too.

The previous owner sent me a photo of the Smog Check Vehicle Inspection Report (VIR) and everything passed in all the categories. The exact date it passed smog was 05/17/21 at 152729 miles. I bought the car at 158,xxx miles btw.

And okay I'll clear the DTC's and see how long it takes for them to come back + inspect everything.
 

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It probably had a cat on it at the time it passed smog and then it was removed afterwards. California is pretty strict with emissions. Like CalgaryTSX said, clear your codes first and then monitor which come back and when they do. Then we can take them one by one. Please do this before you start replacing anything.

You likely have to install the suggested O2 sensor defouler to avoid the DTC but that may not be acceptable to pass the smog test in California. I've heard of others using one but that was in conjunction with a high-flow cat and that was said to have passed a California smog test. Your mileage may vary depending on the test site.

One of those codes refers to a possible battery/charging system issue. If you have a multimeter take a look at your charging system health.
 
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