Black O2 Sensor?

[Homeowner A]

I've been having misfire issues with my UTV 400 when it gets warm and I've thrown all kinds of parts at it so far with no permanent resolution.  Today I removed the O2 sensor and it is black on the end.  To me that means it's running rich and/or the sensor isn't working.  I replaced the sensor with a new one and will see how it goes I guess.  With EFI, I'm wondering why the sensor would be this way with low hours and mileage?  Is there a way to adjust or lean out fueling on these?  Thanks in advance.  

[aefron88]

They tend to run rich and foul plugs and sensors as do a lot of small engines in general. in some versions of the manual it actually recommends periodically cleaning the O2 sensor, but I suspect they removed it from a lot of the retail ones because the EPA doesnt want that to be a regularly needed maintainence item, and there is some risk of damaging the sensor.

If you look at my post below it explains how to hook up a scanner so you can see what each sensor is reading. you could alternatively test this with a "fast" volt meter directly on the O2 sensor. It's a narrow band O2 sensor so it SHOULD be reading back and forth between O V and 1 V as it trims fuel and adjusts between rich and lean. Sticking to one voltage or the other would indicate that it's not reading properly. Again you need a fast (as in multile accurate readings per second) digital volt meter to accurately read that. In HUD ECU Hacker below you should also see Short/Long term fuel trims as being big numbers if the computer is having to agressively compensate for rich/lean running conditions, or a failed sensor that makes it think thats the case.