Will a Bad O2 Sensor Always Throw a Code?

Breto31

Breto31

TJ Enthusiast
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Nov 30, 2020
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Kentucky
2005, 3.73 gears, 6-speed, 33x10.50x15. I have changed to the correct Speedo gear as well…. My mpg has routinely been 10-12 over the last few years. It’s not my DD so I haven’t been too concerned, but lately I’ve been on a couple of 200 mile round trips, so I’ve paid more attention.

Jeep runs great, and has no codes. Just curious if the O2 sensors are bad would I be getting a code - OR - could they be bad and I’m just not being notified?
 
They won't always throw a code. Sometimes they will just cause the ECM to adjust the fuel ratio incorrectly. However, with 33" tires, your mileage is probably because of your differential ratios. With 33s, you should be running at least 4.27 or 4.56. More experienced heads will know for sure.
 
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Breto31 said:
Jeep runs great, and has no codes. Just curious if the O2 sensors are bad would I be getting a code - OR - could they be bad and I’m just not being notified?
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No, the PCM isn't always smart enough to always know when an O2 sensor is bad. It can be bad and the only symptom would be reduced mpg and/or poorer engine performance with no DTC code thrown. And you can have a good O2 sensor when getting a bad O2 sensor code which can be caused by a bad catalytic converter. I had that one for a while on my present TJ, it kept misfiring and sending a bad O2 sensor code but it was ultimately found to be a bad pre-catalytic converter.
 
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C.J. Roberts said:
They won't always throw a code. Sometimes they will just cause the ECM to adjust the fuel ratio incorrectly. However, with 33" tires, your mileage is probably because of your differential ratios. With 33s, you should be running at least 4.27 or 4.56. More experienced heads will know for sure.
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4.56 is my recommendation for 33's with the 4.0 engine and manual transmission. I've never seen a 4.27 listed for a Dana 30.
 
This helps! I just thought 11 was low, even for bad gearing. I’m not going to regear anytime soon, and I know my gears aren’t ideal for 33’s. That’s a future project.

Thanks for the replies!
 
I know everyone is saying 4.11s are low for 33s… but I run 4.11s on 37s (lightweight set with aluminum wheels) and while I know I use fourth for any power, I have no lugging issues and have enough power in 5th gear for small hills and enough to accelerate decently on flat ground. I have 4.0, with a throttle body spacer (not sure if useful at all bought before I knew anything…I’ve had this Jeep 21 years), and cold air intake fed through a snorkel (which does make a significant difference in intake temp…monitored with the OBD II port). Final thing is I did replace the ignition coil with the viper coil. Doubt any of that is enough though to make a huge power difference. I’ve even had some well knowledgeable people drive it and say it feels fine…I get 13-15mpg on the highway at 65-70…maybe that’s bad, but for all my mods and weight and tire size, I’m happy with it - well debating moving up to 4.56s to get more use out of 5th, but there is no way I would go more than 4.88s (which is close to driving in 4th as I’m geared now, and I don’t like that).
 
Brian.souhan said:
I know everyone is saying 4.11s are low for 33s… but I run 4.11s on 37s (lightweight set with aluminum wheels) and while I know I use fourth for any power, I have no lugging issues and have enough power in 5th gear for small hills and enough to accelerate decently on flat ground. I have 4.0, with a throttle body spacer (not sure if useful at all bought before I knew anything…I’ve had this Jeep 21 years), and cold air intake fed through a snorkel (which does make a significant difference in intake temp…monitored with the OBD II port). Final thing is I did replace the ignition coil with the viper coil. Doubt any of that is enough though to make a huge power difference. I’ve even had some well knowledgeable people drive it and say it feels fine…I get 13-15mpg on the highway at 65-70…maybe that’s bad, but for all my mods and weight and tire size, I’m happy with it - well debating moving up to 4.56s to get more use out of 5th, but there is no way I would go more than 4.88s (which is close to driving in 4th as I’m geared now, and I don’t like that).
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Just a note…I’m the black sheep of the forum as everyone tells me everything I’ve done is wrong, yet the Jeep drives well to me (and as a former Jeep engineer said…it drives better than he expected 🤣🤣🤣)
 
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Brian.souhan said:
I know everyone is saying 4.11s are low for 33s… but I run 4.11s on 37s (lightweight set with aluminum wheels) and while I know I use fourth for any power, I have no lugging issues and have enough power in 5th gear for small hills and enough to accelerate decently on flat ground. I have 4.0, with a throttle body spacer (not sure if useful at all bought before I knew anything…I’ve had this Jeep 21 years), and cold air intake fed through a snorkel (which does make a significant difference in intake temp…monitored with the OBD II port). Final thing is I did replace the ignition coil with the viper coil. Doubt any of that is enough though to make a huge power difference. I’ve even had some well knowledgeable people drive it and say it feels fine…I get 13-15mpg on the highway at 65-70…maybe that’s bad, but for all my mods and weight and tire size, I’m happy with it - well debating moving up to 4.56s to get more use out of 5th, but there is no way I would go more than 4.88s (which is close to driving in 4th as I’m geared now, and I don’t like that).
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If all you know is 37’s and 4.10, then how can you determine that 4.88 or more would be too crazy? I didn’t like 4.10 on 33’s, no way in hell I’d kick that up to 37’s and 4.10. I’d be at 5.13 on a Jeep limited to it and 5.38 on a rubicon if I wanted 37’s.
 
The ratio of 4th to 5th is similar (although far from exact) to the ratio of 4.88 to 4.10s, so driving in fourth is like driving in 5th for 4.10s. For me I think the ideal is 4.56. Now I do have the atlas 4 speed, so wheeling I have an incredible selection of ratios with a doubly low around a ridiculous 160:1.

Not saying right for everyone, but know I was happy with the 4.10s on 33s. While not much, I do think I did get some power gain with the cold air and viper coil…
 
Brian.souhan said:
The ratio of 4th to 5th is similar (although far from exact) to the ratio of 4.88 to 4.10s, so driving in fourth is like driving in 5th for 4.10s. For me I think the ideal is 4.56. Now I do have the atlas 4 speed, so wheeling I have an incredible selection of ratios with a doubly low around a ridiculous 160:1.

Not saying right for everyone, but know I was happy with the 4.10s on 33s. While not much, I do think I did get some power gain with the cold air and viper coil…
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Most people here wouldn’t even want 4.88. They would want 5.13 or 5.38. And there’s way more to it than just driving in 4th as a comparison. 4.56 would for sure be disappointing to spend all the money on. You may as well stick with the 4.10 forever if your potential regear plan would be 4.56.
 
Yes, great point! Driving in 4th only shows you the difference at high end, not acceleration and starting from a stop…so agree. Since I drive mine almost as a daily driver spring - fall (criteria, warmer than 25 morning, 55 afternoon, less than 40% chance of rain), I see a lot of flat highway driving. While I know from research that driving at 2600-2800 rpms is fine for the 4.0, I prefer 2000-2200 rpms. Just feels less harsh. Granted being at sea level and the few mods I did do helps, compared to many who are at higher elevations and different weather conditions.

And yes, you hit my current dilemma as well, I know the 4.56s would be perfect for me, but the cost for such a small change is offf-putting! As I know I will never go larger I’m hit with that dilemma…so unless I get some play money, I’m stuck at 4.11s.

I’ve also done numerous mpg tests, but really hard to get good equivalent data on highway sections, but right now I don’t see a noticeable difference at highway speeds between 4th and 5th. (Really really hard to pull out statistical signifigence…did three runs each gear).

But we have hijacked this thread, so will leave it at that unless we start another one as I enjoy the discussion and everyone’s opinions and what works for them!
 
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