P0420 code and check engine light

AaronPaul

TJ Enthusiast
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I will try to make this short and sweet. Last week I filled up my 2000 Jeep TJ and minutes later the check engine light came on. The code that came back was P0420 Bank 1. The check engine light remained on all week until I got gas today and, boom, the check engine light is gone. I ran my OBD scanner again and it said I had 1 pending code P0420 but was that from the previous issue or is there still an issue? I cleared the code from my OBD, ran a diagnostic again and it came back with no codes. My first question is in regards to the OBD scanner, did I do this correctly or incorrectly? 2nd, why on earth would I have a Catalyst Bank 1 code and then fill up and it goes away??? Anyone care to help? Thank you
 
You have an O2 sensor before and after the cat. Once you're in closed loop mode (warmed up engine), the before one should fluctuate and the after one should remain fairly steady.

If the computer senses that the signals are out of wack, it will throw this code. It could mean a bad cat, but that's relatively rare and you would probably notice it (rotten egg smell, sluggish performance). I have heard of people using an infrared thermometer to check cat temperature.

O2 sensors are a wear item (80-100K before replacement), so I'd recommend replacing them. They are important to peak engine performance (fuel/air mixture), drivability, and MPG. I did all 4 of mine recently and the hardest part of the job was accessing and disconnecting the wiring harnesses.

Make sure to read this thread before you do the job:

https://wranglertjforum.com/threads/the-official-jeep-wrangler-tj-oxygen-o2-sensor-thread.4068/
 
You have an O2 sensor before and after the cat. Once you're in closed loop mode (warmed up engine), the before one should fluctuate and the after one should remain fairly steady.

If the computer senses that the signals are out of wack, it will throw this code. It could mean a bad cat, but that's relatively rare and you would probably notice it (rotten egg smell, sluggish performance). I have heard of people using an infrared thermometer to check cat temperature.

O2 sensors are a wear item (80-100K before replacement), so I'd recommend replacing them. They are important to peak engine performance (fuel/air mixture), drivability, and MPG. I did all 4 of mine recently and the hardest part of the job was accessing and disconnecting the wiring harnesses.

Make sure to read this thread before you do the job:

https://wranglertjforum.com/threads/the-official-jeep-wrangler-tj-oxygen-o2-sensor-thread.4068/
The Jeep has 48k on it so I dont think its a bad cat but, it is a 21 yr old vehicle so who knows. Performance seems normal, no rotten egg smile but if you listen to the tail pipe only while its idling there is a bit of sputter to it. Its just odd the check engine light went off
 
An exhaust leak can also trigger this code. Your profile doesn’t say what engine you have but on my 4.0 with 109K, all of the manifold to cat connections were loose. Might be worth checking.
 
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An exhaust leak can also trigger this code. Your profile doesn’t say what engine you have but on my 4.0 with 109K, all of the manifold to cat connections were loose. Might be worth checking.
Awesome thank you, I will check that as well. There is only 48k on it but it is 21 years old. I just bought it 6 weeks ago. No problems at all till 2 weeks ago, filled up and minutes later the check engine light came on and I got that code. Filled up again yesterday, check engine light went off. I'm a bit lost . Yes its 4.0
 
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Pending means there is potentially still an issue, and the pcm is waiting to see if it clears during it's next test.

My experience is that 98% of the time a p0420 or p0430 sets..... you need a converter.

The other 2% of the time there is an exhaust leak either right before or after the post cat O2. This squews the post cat O2 sensor into thinking the cat is bad.

I have never had a P0420/P0430 corrected by replacing an O2 sensor.
 
Pending means there is potentially still an issue, and the pcm is waiting to see if it clears during it's next test.

My experience is that 98% of the time a p0420 or p0430 sets..... you need a converter.

The other 2% of the time there is an exhaust leak either right before or after the post cat O2. This squews the post cat O2 sensor into thinking the cat is bad.

I have never had a P0420/P0430 corrected by replacing an O2 sensor.
I cleared the code because the check engine light went off. I then put the OBD back in and ran a diagnostic and nothing came up as far as codes. Thoughts?
 
I cleared the code because the check engine light went off. I then put the OBD back in and ran a diagnostic and nothing came up as far as codes. Thoughts?
That is because you cleared the code, now you have to wait for I/M system monitor tests to be performed.
This may take a series of drive cycles and a certain parameters need to be met before the catalyst monitor test is run.
 
That is because you cleared the code, now you have to wait for I/M system monitor tests to be performed.
This may take a series of drive cycles and a certain parameters need to be met before the catalyst monitor test is run.
Ok awesome, than you. The only thing I am confused on is why the check engine light went off.
 
Ok awesome, than you. The only thing I am confused on is why the check engine light went off.
It may have passed the catalyst monitor during the last drive cycle.

Check and make sure you do not have and exhaust leak in the area of the post cat O2.
Check and make sure the O2 is not loose.

You can continue to drive your Jeep, but if the code resets the catalytic converter has degraded.
 
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It may have passed the catalyst monitor during the last drive cycle.

Check and make sure you do not have and exhaust leak in the area of the post cat O2.
Check and make sure the O2 is not loose.

You can continue to drive your Jeep, but if the code resets the catalytic converter has degraded.
Will the code show up again? Check engine light? Thank you
 
my tj and my wifes renegade got this code before, i checked several resources and i found general answer, replacing cat. converter is the best way (cost about $900 per vehicle) . now they are happy and i am happy, maybe my wallet is not but, after 3 years there is no obd error code like p0420.