[archer75]

Could I have a bad O2 sensor if I have no check engine light, no codes and just passed an emissions test? Just trying to figure out why I get bad mileage(around 11mpg, all city driving) and if it could be improved. I have 81k miles on an 04 LJ.

[Texan4life]

I get the same mileage on my 04 LJ with a bad O2 sensor, but I have the MIL and a P0432 code. With a MIL, the engine goes to "Open Throttle Mode" and runs rich. You should get a code if something is wrong. To check the codes turn the ignition switch ON-Off-ON-OFF-ON within 5 seconds. If you have a fault code, it will appear on your odometer. Some people change out their O2 sensors as regular maintenance to help fuel efficiency. I've only ever had problems with Bank 2. Jeep fixed it twice under the 80k federal emissions warranty. Once at 34k and again at 68k. Now at 113k, it has given me the same problem. Jeep replaced the precats and O2 sensors each time. I hope to fix it with just the O2 sensors. 1 pair cost me $79 at Auto Zone.

I'd start with air filter, spark plugs, and fluid changes.

[archer75]

Originally Posted by Texan4life

I'd start with air filter, spark plugs, and fluid changes.

Check, check and check.

Still no codes.

I checked autozone and see both 02 pre cat sensors for $25 each so I may grab those just because.

[Texan4life]

I hear that if the precat and postcat sensors are different ages/brands that they may be more likely to cause fault codes. The postcat sensors cost about $45-50.

[archer75]

Originally Posted by Texan4life

I hear that if the precat and postcat sensors are different ages/brands that they may be more likely to cause fault codes. The postcat sensors cost about $45-50.

Hmm, I think they are all originals. But I will double check.

[Slithering_Joe]

Originally Posted by archer75

Could I have a bad O2 sensor if I have no check engine light, no codes and just passed an emissions test? Just trying to figure out why I get bad mileage(around 11mpg, all city driving) and if it could be improved. I have 81k miles on an 04 LJ.

If my wife only drives her Liberty in the city, the computer will state an approx 15 to 16 mpg trip. On the highway it will jump to 19-21 mpg. You can have a slow responding O2 sensor that can bring down your mileage but still be within operation specification. If the sensor truely is slow and your mileage has decreased due to it, a check engine light is around the corner. When replacing O2 sensors, I only go with OE (Mopar) from the dealership. Don't bother replacing the after-cat/downstream O2 sensor until a code related to that is set. The downstream O2 sensor have no impact on mileage.

[archer75]

Originally Posted by Slithering_Joe

If my wife only drives her Liberty in the city, the computer will state an approx 15 to 16 mpg trip. On the highway it will jump to 19-21 mpg. You can have a slow responding O2 sensor that can bring down your mileage but still be within operation specification. If the sensor truely is slow and your mileage has decreased due to it, a check engine light is around the corner. When replacing O2 sensors, I only go with OE (Mopar) from the dealership. Don't bother replacing the after-cat/downstream O2 sensor until a code related to that is set. The downstream O2 sensor have no impact on mileage.

I bought it used. So if there has been a decrease in mileage over what it was I wouldn't know it.

[tlendt]

An O2 sensor can have brutally slow cross counts and wild voltage swings and still not set a code. BTW last time I checked Mopar wasn't manufacturing O2 sensors. They buy them, just like we do. Its the same part for twice the money.

[barty]

Bosch, Denso and NGK make virtually all of the OEM O2 sensors out there. If you buy any one of those brands you're getting virtually the same thing that came out of the vehicle. Some replacements I've bought were virtually IDENTICAL to the OE sensor, right down the Bosch logo stamped on the case.

I replace mine on vehicles at ~100k as a maintenance item. While they may not have failed, they're certainly less efficient than they were when new. A heated, 4 wire sensor should last at least that long.

I would only replace the after-cat sensor if it trips a code. Its only purpose is to monitor converter efficiency and has no effect on mixture control.

[Slithering_Joe]

Originally Posted by tlendt

... BTW last time I checked Mopar wasn't manufacturing O2 sensors. They buy them, just like we do. Its the same part for twice the money.

I'm quite aware of that.

Originally Posted by barty

Bosch, Denso and NGK make virtually all of the OEM O2 sensors out there. If you buy any one of those brands you're getting virtually the same thing that came out of the vehicle. Some replacements I've bought were virtually IDENTICAL to the OE sensor, right down the Bosch logo stamped on the case.

I replace mine on vehicles at ~100k as a maintenance item. While they may not have failed, they're certainly less efficient than they were when new. A heated, 4 wire sensor should last at least that long.

I would only replace the after-cat sensor if it trips a code. Its only purpose is to monitor converter efficiency and has no effect on mixture control.

My experience with Bosch aftermarket has been very dissapointing. Also, in many cases, the tolerances followed by parts manufacturers can/may be more demanding when they supply the car manufacturers branded parts compared to the aftermarket chain stores. To be safe, many times I choose the OE parts.

[tlendt]

So by that rationale can we assume you buy your spark plugs from the dealer too? I go through a lot of Bosch O2's from the parts store (and spark plugs) and can't remeber ever having an issue with either.

[Slithering_Joe]

Originally Posted by tlendt

So by that rationale can we assume you buy your spark plugs from the dealer too? I go through a lot of Bosch O2's from the parts store (and spark plugs) and can't remeber ever having an issue with either.

I'm not making this into an argument of where to buy your parts. I provided a fact of where I get my O2 sensors from. I didn't say where you or anyone else should get them from nor did I correct someone in their choice. Since you sell so many parts,

• do you keep records on the person who buys the part,
• research to see how long the part lasts/lasted,
• was it installed correctly,
• was it the correct part,
• did it really need that part,
• ever drove the car after the said part was installed,
• wonder if the parts people are buying now aren't replacing the parts you sold before??

So how can you say you for sure you don't have any issues?

I'm basing my choices on experience from the past and there are some parts I will not buy from certain brand names. By the way, I have NGK plugs in the Wrangler and Mustang, Ford Motorcraft filters & Mobil oil in the Mustang, Wrangler & Liberty. When it comes to the O2 sensors, I'll buy from the dealership. When or if I get around to changing a water pump for example, I'll install a Motorcraft in the Ford and Mopar in the Jeeps. My choices are based on what parts I think (hint: my opinion) or have experienced to be the best. Sometimes the OE part is best, sometimes the aftermarket is best, and sometimes the aftermarket is the exact same part as the original equipment (OE). Other times, the opposite can happen as well.

[Slithering_Joe]

And another thing... when I did brakes on the Mustang years ago I had to replace the junk crap ___ OE (Kelsey-Hayes) rotors that were on it. I replaced them with Brembo ones for two reasons: (1) OE ones sucked and many other owners were having problems with them too, (2) Brembo was much cheaper and have a great reputation. ($180 OE vs $60 Brembo, each). After a few model years have gone by, Ford's OE supplier is now Brembo but I still won't buy them from the dealer becasue they are still expensive. Go figure. I haven't had a problem since.

[tlendt]

I've been a vocational auto mechanics instructor for 17 years. I live in Iowa where the widespread use of poorly blended ethanol mixed fuels leads, at least in part to a lot of failed O2 sensors. We go through a lot of them. Never had one fail to work properly right out of the box, and we pull a data stream before and after on every emission/driveability complaint. I would have to strongly advise my students not to buy these from the dealer unless its the last resort, or the customer specifically requests it (usu. the same guy that demands Pennzoil) Apparently my experience is different than yours, but it is experience.

[jgorm]

Originally Posted by Slithering_Joe

And another thing... when I did brakes on the Mustang years ago I had to replace the junk crap ___ OE (Kelsey-Hayes) rotors that were on it. I replaced them with Brembo ones for two reasons: (1) OE ones sucked and many other owners were having problems with them too, (2) Brembo was much cheaper and have a great reputation. ($180 OE vs $60 Brembo, each). After a few model years have gone by, Ford's OE supplier is now Brembo but I still won't buy them from the dealer becasue they are still expensive. Go figure. I haven't had a problem since.

I bought my mustang brembo brakes (00 cobra R) from ford racing for $1100 and they were 2500 from brembo directly.
I tune mustangs as a side job and have seen may customers have problems with non motorcraft brand o2 sensors. Sometimes they make the tuning nearly impossible and i instruct them to replace their brand new knock off sensor with the oem version and everything comes into line like it should. Not sure if jeeps are the same, but it does make a huge difference on fords.
The o2 sensor will not throw a code if it is lazy, or less than 25% off. Its a good idea to replace them every 30K mi because they are the most important sensor on the motor.