Will replacing my o2 sensor improve fuel economy / MPG?
 

So I've been doing some mods on my 2004 civic DX 5-speed, but I've been running with a dead and disconnected o2 sensor for... 100000 km?

I'm currently getting about 39 mpg as it is, but would a working o2 sensor on the civic significantly improve that number? The o2 sensor for this car is ~$170 CAD ($93 USD), so it's not a cheap fix. It's worth doing if I'll get a significant improvement, as I travel 150km a day for work, but not if it'll only give a minor boost.

What's your experience?

I would say yes.

A bad O2 sensor on my first Firefly caused it to run rich (open loop). It also may have been the root cause of a ruined catalytic converter (dumping fuel on the fire burned it out and the internals disintegrated).

I didn't get any before/after numbers though, because I had to fix the O2 sensor to register it in the first place.

I have no experience with Civic's, but In general, the O2 sensor is primary feedback sensor that the ECU uses to determine mixture in closed loop. Without it you car is probably staying in open loop and running on its default programming like it does on initial start up. Computer can't optimize fuel delivery or timing and you could do damage to other parts like MetroMPG said. I have no idea how much it's hurting your FE but it certainly is. I assume your CEL is on. If money is really tight you could maybe try one from a wrecker if you can find one with a good return policy. If you put it in and it works (no CEL) your ahead, if not take it back. Just a thought. I should ask, is it the front or back one that is disconnected? The back one usually just monitors the cat, but the front one is really important.

It depends on if it's the upstream or downstream O2 sensor. The car uses the upstream (before catalytic converter) to adjust air / fuel mixture, but the downstream (after cat) sensor just measures the function of the cat. The upstream sensor is the one that matters.

You can buy aftermarket sensors for much less and install it yourself.

Upstream
 

Ok, so it sounds like I should go to a wrecker and try a few. It is the upstream sensor, so it's likely to make a difference. It'll likely be a week or two before I get to it so I'll have a better baseline comparison for before and after.

No, you should go to an auto parts store and get a new one. O2 sensors are a consumable and don't last forever. They age and wear out. One from a wrecker isn't going to work as well as a new one if it works at all.

Exchange
 

Our wrecker has a return policy if it doesn't work.

Again I will mention that they age. If you are trying to see a difference, how are you going to know if it works or not? It could be working at anywhere from 1% to 90% capacity if its from a junkyard.

Do not get one from a wrecker...

An old O2 sensor makes your engine run slightly richer. That is, they read a little bit leaner as they age. How do I know? My old oil burning car chewed through O2 sensors very quickly, and towards the end of the sensor's life the exhaust had a noticeable unburned fuel smell.

It's 25 freaking dollars, just buy a new one.

Seconded! Unknown history, loads of dirst and moisture, potential for coolant or oil contamination... Get the correct part and install it carefully. Both the environment and your engine will be far better off.

Simon