2005 Rubicon - P0300 and P0431

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mashenden

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Richmond, VA, USA
In case anyone experiences something similar, our 2005 Rubicon w 4.0 L started running rough and threw a check engine light. A quick detour to a parts store established codes P0300 (multiple cylinder misfires) and P0431 (catalyst eff below threshold).

Given the jeep only has 38K miles (I know :) ) I was skeptical that new plugs would solve the issue, but so far, so good. No codes have returned.

It surprised me it needed new plugs already, but after some research on this forum I learned that the 4.0L engines, 2000 and newer, have the DIS ignition system. And this type of ignition is hard on conventional copper-core spark plugs. (thank you @JerryBransford).

Unfortunately I learned of this after the plugs were replaced with the same type that were in there, so it looks like we will only be good for another 40K miles +/-. Its not worth worrying about now but I do wish I had gone with dual-tipped Platinum or Iridium (XP985) spark plugs as noted in other posts.
 
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I hope I'm wrong, for your sake, but I've never heard of spark plugs solving a P0431 code, and that one takes some time to show up (if you cleared the code), so it may just be a matter of time. The common causes of a P0431 code are a bad catalytic converter, a bad O2 sensor, or an exhaust leak. There are other causes, but they don't happen very often. What triggers that code is the computer detecting that the switching rate (the conversion process creates a cyclic rich-lean-rich-lean exhaust stream) of the downstream sensor slows down below a threshold, which indicates that conversion is not taking place (the converter is no longer working.)

One common way to test the catalytic converter (at least in other vehicles) is to use an IR temp gun. Once up to operating temperature, the outlet of the cat should be significantly hotter than the inlet (the conversion process is exothermic and creates energy). However, I've not tried it, yet, on my LJ, and that may not work due to the proximity of the cats to the exhaust manifold. Usually, the cats are farther away from the cylinders, and the exhaust cools significantly before getting to the inlet of the cat.
 
sab said:
I hope I'm wrong, for your sake, but I've never heard of spark plugs solving a P0431 code, and that one takes some time to show up (if you cleared the code), so it may just be a matter of time. The common causes of a P0431 code are a bad catalytic converter, a bad O2 sensor, or an exhaust leak. There are other causes, but they don't happen very often. What triggers that code is the computer detecting that the switching rate (the conversion process creates a cyclic rich-lean-rich-lean exhaust stream) of the downstream sensor slows down below a threshold, which indicates that conversion is not taking place (the converter is no longer working.)

One common way to test the catalytic converter (at least in other vehicles) is to use an IR temp gun. Once up to operating temperature, the outlet of the cat should be significantly hotter than the inlet (the conversion process is exothermic and creates energy). However, I've not tried it, yet, on my LJ, and that may not work due to the proximity of the cats to the exhaust manifold. Usually, the cats are farther away from the cylinders, and the exhaust cools significantly before getting to the inlet of the cat.
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Yep, I hope you are wrong too. A bad cat at 38K would be unusual. I think (hope) that the misfiring caused incorrect readings. Its plausible that misfires could cause a rich-lean-rich-lean cycle. Anyhow, time will tell. If I find out otherwise, I will come back and update this.
 
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mashenden said:
A bad cat at 38K would be unusual.
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Yes, it would be! However, it happens. An injector stuck open, dumping raw fuel into the exhaust stream, will kill a catalytic converter, but if you're issue was brought on suddenly, I doubt that would be the case.

mashenden said:
Its plausible that misfires could cause a rich-lean-rich-lean cycle.
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True, but this would generally trigger a different code because the upstream O2 sensor would detect it because it was happening prior to the catalytic converter.

Like I said, my fingers are crossed for you!
 
mashenden said:
Yep, I hope you are wrong too. A bad cat at 38K would be unusual. I think (hope) that the misfiring caused incorrect readings. Its plausible that misfires could cause a rich-lean-rich-lean cycle. Anyhow, time will tell. If I find out otherwise, I will come back and update this.
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sab said:
Yes, it would be! However, it happens. An injector stuck open, dumping raw fuel into the exhaust stream, will kill a catalytic converter, but if you're issue was brought on suddenly, I doubt that would be the case.



True, but this would generally trigger a different code because the upstream O2 sensor would detect it because it was happening prior to the catalytic converter.

Like I said, my fingers are crossed for you!
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Good points, no doubt. Thank you for the insight. It is definitely appreciated.

It was a sudden change from running good to throwing the codes and then running rough.

I too was skeptical that plugs would solve the issue. I suggested fuel injectors first, but the shop felt plugs were warranted (unfortunately ours Jeeps seem to develop issues in FL when I am in VA - grrrr). I also thought maybe it was the ignition pack since P0300 is misfires in multiple cyclinders.

So far 140 miles and no check engine light.
 
mashenden said:
Good points, no doubt. Thank you for the insight. It is definitely appreciated.

It was a sudden change from running good to throwing the codes and then running rough.

I too was skeptical that plugs would solve the issue. I suggested fuel injectors first, but the shop felt plugs were warranted (unfortunately ours Jeeps seem to develop issues in FL when I am in VA - grrrr). I also thought maybe it was the ignition pack since P0300 is misfires in multiple cyclinders.

So far 140 miles and no check engine light.
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If it's being driven back to VA and the P0431 returns, with no noticeable performance issue (like running rough), there's no harm (other than to air quality from the increased emissions) in continuing to drive it back to VA, where you can work on it.
 
P0431 is a very common code to throw when there's a big change in ambient (weather) temps. I have no idea why, but whenever we go from 50's at night to 80s the next day, I throw that code. Usually once or twice every spring and fall. I clear it, it stays off for thousands of miles, comes back when the seasons change again (I live in VA). Unless P0431 regularly and rapidly returns, I would always clear it and forget it. As for the multiple misfire- I tracked down a random p0300 and then p0301 (also felt the misfiring) for a year and a half, changing just about everything related. In the end I found a thread about valves slightly rotating above 3200 rpm and getting stuck on carbon deposits (there's a TSB about this, even at lower mileages). If i get a misfire, I drive it hard for a couple of miles (to reposition the valves) and it always goes away. Tends to happen to me on hot highway drives a couple of times each summer, which Is the only time I tend to get into 3000+ RPM range.
 
sab said:
If it's being driven back to VA and the P0431 returns, with no noticeable performance issue (like running rough), there's no harm (other than to air quality from the increased emissions) in continuing to drive it back to VA, where you can work on it.
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alpineovernappa said:
P0431 is a very common code to throw when there's a big change in ambient (weather) temps. I have no idea why, but whenever we go from 50's at night to 80s the next day, I throw that code. Usually once or twice every spring and fall. I clear it, it stays off for thousands of miles, comes back when the seasons change again (I live in VA). Unless P0431 regularly and rapidly returns, I would always clear it and forget it. As for the multiple misfire- I tracked down a random p0300 and then p0301 (also felt the misfiring) for a year and a half, changing just about everything related. In the end I found a thread about valves slightly rotating above 3200 rpm and getting stuck on carbon deposits (there's a TSB about this, even at lower mileages). If i get a misfire, I drive it hard for a couple of miles (to reposition the valves) and it always goes away. Tends to happen to me on hot highway drives a couple of times each summer, which Is the only time I tend to get into 3000+ RPM range.
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LONGJP2 said:
Check the mini-cats mounting bolts.
They're known to come loose.
An air leak can cause the 0421 & 0431.
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Good to know. Thank you all.

BTW, it is not a VA jeep, but instead lives in the Keys, where I visit frequently. It made it back there after replacing the plugs. There was a notable cold spell when it decided to act up. That may be a consideration as will the loose cat if it throws that code again.
 
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