I was just trying to test the O2 Sensor on my 1990 Honda Civic and I wanted to see the difference between it connected and disconnected while driving...So I turned on the car and let it heat up for like 15mins, then disconnected the O2 Sensor plug and noticed nothing happened noticeably...So then I went for like a 20 minute drive...Car ran fine, no hiccups nothing...What I thought was odd though was that the check engine light NEVER came on the whole time driving or even idling...I even checked for a code on the ECU after and there was nothing...Is something wrong here?

DJ

 

My '90 Civic Si didn't trip a code either when I tested my O2 sensor that way. I had surging most noticeable at about 25mph in 3rd gear. Pulling the plug made it goe away. Long story short, my O2 sensor was slow and lazy and replacing it with an OEM sensor cured that problem. The cheaper generic Bosch sensors didn't work well or for too long. I would think an open sensor would set a code, but mine didn't. Beats me why.

 

The Honda ECU is designed to be able to run with-out an o2 if it has to. It will just run extremely rich.

 

So does that mean if I dont feel any difference and am getting low MPG and power that it could be the O2 sensor? Cuz I dont feel any diff with it on nor off

 

The computer won't run extremely rich with the O2 disconnected, in fact the reverse will happen - it will lean out. That is, until the computer figures out the O2 is disconnected, at which point it will revert to the safe fuel maps and trigger an O2 sensor code.

Trust me on this, I drove without an O2 for about 3 months, and yes I have an A/F gauge attached (I just disconnected the pinout from the ECU). Mileage didn't suffer and neither did power.

The only real way to trigger an O2 sensor code on single-wire O2s is to find a big hill, put it in gear, and then just let the car roll down the hill, engine braking the entire way (do NOT press the gas at all). After about 10-20 seconds the computer will finally figure out the signal's not going lean, and it will trigger the code. Try this, I'm sure it'll work to get you your code


[Modified by raene, 3:45 PM 2/4/2003]


[Modified by raene, 3:46 PM 2/4/2003]

 

raene... Your post make sense regarding the engine braking downhill test. Thanks for that insight.

As far as power loss is concerned, there most likely would be none since the ECU ignores the O2 sensor input at WOT and reverts to the programmed fuel map.

I would disagree with you about the 'mileage didn't suffer' part, though. I realize your statement is based on your own experience, but mine and generally other's experience is that overall mileage will decrease slightly with a malfunctioning O2 sensor. Specifically in my case, mileage dropped almost 2 miles per gallon.

 

Did you have an O2 code during that period?

You may be right, I meant no real mileage loss in that I didn't find any increase/decrease in mileage to be noticeable If I'm +/- 20 km's per tank or whatever I tend to attribute that more to how I've driven during that period than to other factors, such as O2 performance.