Originally Posted by Ed@Ford
Heres the deal on two O2 sensors on a CRF. Go to the www dot rickramsey dot net web page and look in the introduction of the CRF250L file. One O2 sensor is in the exhaust port on the cylinder head. It looks at O2 level in the exhaust gas. The other sits on the rocker cover box and it looks at O2 in the fresh air that is pumped into the exhaust flow. I do not know if either sensor is heated, or if either is a "wide ratio" sensor. Some of this can be "guessed" if the wiring harness pinout for these sensors are are known. However, since the air induction sensor is looking at fresh air, I'd guess that is a wide ratio sensor. Based on past experience, this seems like an expensive solution to control the metering of air pumped into the exhaust to control hydrocarbon emissions. I am unaware of this method being used on modern engine emission management systems.....but maybe someone out there is more up-to-date on such methods.
There's only one O2 sensor, and thus at the exhaust port.
The title is a little confusing where it's placed, though ...