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I have awful fuel mileage on my 2002 4dr LX Auto Civic with 117K miles on it. We bought back in February, and I don't remember it being terrible, because the first tank was driven on the interstate back home, but then after that, the wife has been driving it and a couple months ago brought it to my attention. We've seen down to about 22-23 in town (it's hardly a town though as far as stop and go is concerned). The maintenance required light is on, but I believe that's different than the check engine light, correct?

During the summer, I replaced the spark plugs and air filter, and saw a bump up to possibly 32 for a tank maybe and then it was back down, but we see anywhere from 25-29 regularly. We did just get 34 on a complete tank on the interstate with no AC running (but that is EXTREMELY rare). I think the best we've seen before that is maybe 31 or 32. Is there anything you guys can think of?

I asked a guy what things are common when you take it to get a "tune-up", and he mentioned: Plugs (possibly wires - but not really), Fuel Filter, Air Filter, and PCV. I asked at Autozone about the Fuel filter and was told it was in the tank (none serviceable) and the PCV wasn't serviceable for some reason (can't remember now). Anyways. I've seen a lot about O2 sensors and have heard they can really affect mileage, but I also would think they would throw a code if they've gone bad.

Let me know if it there's anything I can try before I replace the O2 sensors (if they can even be replaced). I'd really like to have some confidence and you guys advise before I just start dropping money in it. Thanks in advance. I really appreciate it.

 
Rep Power: 812

seems normal, i usually get 22-24mpg with alot of maintenance. highway driving you should be seeing much higher like 38-40. these cars dont do well in the city. also mileage in the winter is much worse (usually 10mpg lower than summer) due to many factors beyond your control. pcv you can clean out, its metal and reusable. 02 sensors do get worse over time and when they are really bad the engine light comes on. but a sensor over 20k miles will not function as a new one would. how does the engine look inside? if the car hasnt really been taken care of that well, it may benefit from an auto-rx.com treatment to clean out the motor.

 
Rep Power: 367

I usually get around 27-29, 33 when lucky, HWY between 36 and 44, but have manual trans.
the '02 civic O2sensor is not bad pricewise (around 50). Mine had to be changed at 60 to 70k, can't remember, so you might get better with a new one.

 
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How difficult are the O2 sensors to replace, and is there a walk through in another thread? Some vehicles have 2x O2 sensors - if this is the case, which is the more important to replace. Thanks again in advance.

 
Rep Power: 367

took me 2 hours to take the darn... thing because of rust, but pretty much straight forward. You will see it when you pop the hood and look at your exhaust.
Just use adequate sealant.
I did not change the rear one.
If you planning on doing more Do it yourself, you can just get a haynes manual in any parts store such as autozone or similar.

 
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Why no need to change the rear one?

 
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Nevermind - I found it. The front one controls mpgs, and the rear one basically just reads the exhaust info. The front is all I'll need. I just thought I may save someone some time. Thanks again, guys.

 
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Last question - I'm assuming (but you know what that does) that this would be the upper (and pre-cat) oxygen sensor for the LX trim that I have. Is that correct?

 
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you want the sensor before the cat. the one in the cat only records emissions info and has no effect on the way the car runs. you are lucky the 01-03 use cheap narrowband sensors around $50. bosch oem type is good. and easy to change with a jack and stand, and possibly a special socket with cutout for the wires if a wrench doesnt work. on the lx, you may actually be able to reach it from the top without jacking the car up. be very careful with the sensor. its a precision piece and very easily damaged by fluids, grease, your fingers, etc. the outside part is also a sensor so handle the whole thing with care. do not let anti-seize gel get on the sensor. you can clean it with water when the outside gets dirty but thats it.

 
Rep Power: 291

You arent worse off than most