Can a bad O2 sensor cause misfire?

Jramos013

Jramos013

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Hi everyone. I recently got 2 cels that poped up. P0037 for low signal 1/2 o2 sensor and P0300 for multiple misfire. The TJ is running fine, no noticeable symptoms. I checked the wiring, everything looks good. Nothing is damage or broken. I order all new NTK sensors. The only recent change was @Wranglerfix pcm about 1000 miles ago. But I don't think thats the problem.

Could the p0300 code be because of the faulty o2 sensor? What else should I look for? Thanks
 
I went through the sensor wire and connection, everything was good there. Did not check the wiring coming from the harness to connector. Will do that later today.
 
Some updates. I replaced the bad o2 sensor the first week of January. Erased the code and everything was fine. Today a cel came back on with a code P0421. So the precats are bad? Is there a possibility it might be something else?

Thanks everyone
 
Any idea how long you've had the misfire code? A misfire problem will send fuel into your exhaust and burn up pre-cats pretty quickly.
 
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Interesting, did not thought about that. The TJ ran with the p0037 and p0300 codes from October until the first week of January when I was finally able to replace the bad sensor. I used the NTK one. I erased the cels after the install and today the p0421 showed up. I actually never felt a missfire when the code was present.
 
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I've had a similar situation before. I got misfire codes, but never felt like anything was wrong so cleared them. Then had a plugged main cat that was caused by my precats disintegrating. Replaced the cats and thought all was good until I got another misfire code - this time persistent. Fixed the misfire problem, then not too long later got the P0421 code (or something similar - I don't remember the exact code). Had to replace the cats again since I didn't take care of the misfire issue soon enough.
 
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You need to look at live data to be sure. The engine needs to be in closed loop and pull up live data for Bank 1 sensor 1 and Bank 1 sensor 2. Hold RPMs between 1200-1700. B1S1 should be switching up and down. B1S2 should be relatively flat. If B1S2 mimics B1S1, the CAT is shot.

Keep in mind, that if you clear the code, it may take a while to set a code again, making you think it was a fluke. This DTC is monitored in a really specific range. This is straight from Mitchell 1: "When monitored: Engine warmed up to 158 degrees, 180 seconds of open throttle operation, at a speed greater that 18 mph, with the engine between 1200-1700 rpm, RPM and MAP vacuum between 15.0 and 21.0 inches of mercury."
 
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LJBean said:
I've had a similar situation before. I got misfire codes, but never felt like anything was wrong so cleared them. Then had a plugged main cat that was caused by my precats disintegrating. Replaced the cats and thought all was good until I got another misfire code - this time persistent. Fixed the misfire problem, then not too long later got the P0421 code (or something similar - I don't remember the exact code). Had to replace the cats again since I didn't take care of the misfire issue soon enough.
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What ended being the misfire problem? when I first had the CEL I dint knew what to make of it since the TJ was running fine. I thought I had to do something with the sensor being bad.
ItchyDong said:
You need to look at live data to be sure. The engine needs to be in closed loop and pull up live data for Bank 1 sensor 1 and Bank 1 sensor 2. Hold RPMs between 1200-1700. B1S1 should be switching up and down. B1S2 should be relatively flat. If B1S2 mimics B1S1, the CAT is shot.

Keep in mind, that if you clear the code, it may take a while to set a code again, making you think it was a fluke. This DTC is monitored in a really specific range. This is straight from Mitchell 1: "When monitored: Engine warmed up to 158 degrees, 180 seconds of open throttle operation, at a speed greater that 18 mph, with the engine between 1200-1700 rpm, RPM and MAP vacuum between 15.0 and 21.0 inches of mercury."
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I think my code reader does have live data, will confirm this tomorrow.


Thanks for the replies!
 
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Jramos013 said:
What ended being the misfire problem? when I first had the CEL I dint knew what to make of it since the TJ was running fine. I thought I had to do something with the sensor being bad.

I think my code reader does have live data, will confirm this tomorrow.


Thanks for the replies!
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Take a screenshot or picture and post it and I'll help you look at it. It'd be good for people to see this. It takes all the guessing out of it.
 
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P0037 is a heater element circuit code for bank 1 sensor 2. That should not be misfire related.
If you have access to a scanner that reads live data, look at the misfire graphic and see what cylinders are misfiring.
X2 what @ItchyDong said, take a screen shot and post it up.
 
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Jramos013 said:
What ended being the misfire problem? when I first had the CEL I dint knew what to make of it since the TJ was running fine. I thought I had to do something with the sensor being bad.

I think my code reader does have live data, will confirm this tomorrow.


Thanks for the replies!
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In my case, I replaced the spark plugs and changed out the coil rail and that fixed the misfire issue. Pure blind luck in my case, though. From what I read, misfires can be caused by a myriad of situations and you can end up throwing a lot of parts at it. There's a lot of good knowledge from members here, so hopefully you can drill down to the misfire cause pretty quickly.

I suggest you fix the misfire issue before you replace cats (if they are actually bad) as long as your Jeep continues to run OK.
 
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Here's some data from a few minutes ago. The Tj was parked with rpm between 1600-2000. Not really sure what to make of it since I have no experience interpreting this kind of data. The app does not have a misfire live data but I will keep checking with other apps.

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From the first two screenshots posted, you can see your failed CAT. Look At O2S1 (upstream bank 1). See how it's switching? That's good. But on the next screenshot, look at O2S2 (downstream bank 1). It almost mimics what the upstream is doing. When the CAT works properly, it filters out the carbon so the downstream sensor won't register rich. It'll be more flatlined. Your Bank 2 CAT also looks wonky, but it's not as bad as Bank 1. Google image search "catalytic converter oscilloscope" and you'll see some good screen grabs of live data to compare to.
 
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ItchyDong said:
From the first two screenshots posted, you can see your failed CAT. Look At O2S1 (upstream bank 1). See how it's switching? That's good. But on the next screenshot, look at O2S2 (downstream bank 1). It almost mimics what the upstream is doing. When the CAT works properly, it filters out the carbon so the downstream sensor won't register rich. It'll be more flatlined. Your Bank 2 CAT also looks wonky, but it's not as bad as Bank 1. Google image search "catalytic converter oscilloscope" and you'll see some good screen grabs of live data to compare to.
Click to expand...
Thanks so much for your help! Will look for sone google vids along with the cat system…. I thought cat should last longer since my tj only has 88,000 miles. The misfire and fuel dumping to the cat could be an explanation of the failure.

So thats that. Now i have to figure out if the missfire is still present. Could the coil pack be bad and not give a cel? I will take out the plugs during the week to inspect them. How long does the p0300 take to come back if it was still present, the p0421 came by itself with no other codes.
 
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