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Old 10-05-2019, 10:25 PM
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P0420 after replacing plug wires and coil pack

09 JK with 172k. I encountered a misfire and the spark plug wires 1 and 3 were arcing back to the cool mounting bolt. Had the coil and wires replaced and now I am getting a P0420 code. Should I try replacing the 02 sensors or is it the cats.

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Old 10-07-2019, 01:30 AM   #2
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Does it miss all the time or does it smooth out after a while?

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Old 10-07-2019, 07:52 AM   #3
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Most likely it’s the cat for bank 1. Sounds like without spark you have a lot of unburned fuel going through the cats which ruins them.

I’d verify the o2 sensors are working. Using a scan tool, monitor o2 sensor pids. Crate a rich condition by doing a full throttle drive. You should see o2 sensors go full rich. Then create a lean condition by pulling off a vac hose. You should see o2 sensors go lean. If the o2s are reacting, you know they are good.

One other thing to check is if the rears mirror the front o2 sensors. If the rears look to close to the front then your cats are toast.
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Old 10-07-2019, 09:27 AM
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Does it miss all the time or does it smooth out after a while?
No. It started to miss when cold. it up a P0300 a couple of times. Popped the hood and watched the 1 and 3 cables arc at the coil side to the mounting bolt. Replaced the coil and the cables and it runs fine now. Then the P0420 sarted coming.
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Old 10-07-2019, 09:28 AM
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Most likely itís the cat for bank 1. Sounds like without spark you have a lot of unburned fuel going through the cats which ruins them.

Iíd verify the o2 sensors are working. Using a scan tool, monitor o2 sensor pids. Crate a rich condition by doing a full throttle drive. You should see o2 sensors go full rich. Then create a lean condition by pulling off a vac hose. You should see o2 sensors go lean. If the o2s are reacting, you know they are good.

One other thing to check is if the rears mirror the front o2 sensors. If the rears look to close to the front then your cats are toast.
Thanks
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Old 10-08-2019, 11:57 AM   #6
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Most likely itís the cat for bank 1. Sounds like without spark you have a lot of unburned fuel going through the cats which ruins them.

Iíd verify the o2 sensors are working. Using a scan tool, monitor o2 sensor pids. Crate a rich condition by doing a full throttle drive. You should see o2 sensors go full rich. Then create a lean condition by pulling off a vac hose. You should see o2 sensors go lean. If the o2s are reacting, you know they are good.

One other thing to check is if the rears mirror the front o2 sensors. If the rears look to close to the front then your cats are toast.
This.

You can also test a cat by inlet and outlet temperature I believe although I haven't done it before. I had two cat failures on my wifes car and the bank 2 sensor mirrored the bank one sensor. Look at the feed in graph mode after the cat is hot and bank one should zig zag up and down and bank two should pretty much be a straight line.

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