Will replacing my o2 sensor improve fuel economy / MPG?

[TheOutdoorAdventure]

Update: Replaced my O2 Sensor on my civic. No noticeable improvement of fuel economy.

[redpoint5]

You can monitor o2 sensor performance if you have a gauge like Torque connected to your car.

I'm surprised a new sensor didn't improve efficiency. Are you sure the new one is connected and functioning properly? Why was the first one dead? It's a little soon for it to be going out, although it's possible.

[TheOutdoorAdventure]

Update: Performance drop occurred, but after replacing spark plugs the performance returns. I'll have to wait until it's warm to test again, but it did not noticeably improve performance.

[davelobi]

You should have had the check engine light lit on the dash with a disconnected/inop O2 sensor. Did the light go away with the new one installed. It is possible there was no preformance gain because even the new sensor isn't doing its job. Could be an open circuit between the sensor and the computer tht reads the values from the o2 sensor. The chec engine code would likely give a hint to the problem such as "out of range", "voltage low", "open circuit", etc. I don't know Honda codes and their verbage but easily looked up. I also don't know Honda well enough to say that the bad o2 sensor would keep the engine running in open loop but I suspect it would.
Your mileage wasn't bad and remember we are now pumping winter formulation gasoline in our cars and economy will drop slightly for that reason alone. Not to mention in the cold weather the engine may be left idling to warm up the car and will take longer to get up to efficient operating temperatures so it can go into closed loop.

[TheOutdoorAdventure]

Yes, I did all the research, ran the codes, disconnected it a year or two ago and have been running it with it disconnected since then. I've been getting as much as 800 km a tank (45 liters) with it like that. I thought I might be able to squeeze a few more kms out with a working one. I put it in the new OEM one, cleared the codes, it runs well and I haven't thrown a new error since. It's working, but there doesn't seem to be significant changes. Jury is out due to 20 below winter conditions here in Canada.

I agree, I'll be waiting until spring.

[teoman]

Generally ecu needs some time to re-train itself.