I have 2005 Elantra. I have no exhaust or vacuum leaks. I've replaced the sensor with a new sensor (Bosch = High Quality). I inspected the wiring harness the best I could (to the point it goes into the firewall) and I don't see any breaks, etc. If I let the car idle, the data looks good. The sensor reads between 0.100 to about .600 millivolts. It fluctuates fairly frequently between these values. I've let the car idle for about 30 minutes and this behavior is seen throughout the duration. However, the moment I take the car on the road (keeping it at a constant speed), it pretty much stays steady at .750 volts (hence the p0140). I have no idea why it does this. Can anyone please help? Thanks in advance.
According to your post the voltage fluctuates at idle, but goes up to .75 volts under load. That actually sounds like the sensor is working to me, if so that means the exhaust is very rich. Ordinarily that should show up as P0172 exhaust too rich, however if there's a problem with the upstream O2 sensor you could get that message instead. I'd check the upstream O2 sensor, replace if necessary.