You should measure the voltage between the O2 sensor and ground with the engine hot and running. You can do this by disconnecting the wire from the sensor to the ECU and connecting your meter. You should see something in the .25 to .5 volt range.
If you are getting good voltage from the sensor, then I am at a loss because the ECU seems to be working and setting the fault when the engine warms and it goes into closed loop.
The only other thing that I can think of is that you may be so lean that the sensor is generating no or very low voltage and the ECU thinks it's dead and is setting the 50% fault code. What happens if you richen the mix a bit?
Falls Church VA
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