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.

  • Welcome to the site @Paul, what sensor are you talking about here? What does code P0140 mean and did you get it before or after the sensor replacement? A bit more background would be useful too. – GdD Apr 30 '18 at 10:50
  • o2 sensor, downstream (i.e., after the cat). Yes, that is why I replaced it and inspected the wiring harness. P0140 = O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 2). – Paul Apr 30 '18 at 11:01

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.

  • Upstream sensor is within range, typical range I see is about 1.8 - 1.9 volts. The car has a wideband o2 sensor for its upstream sensor. – Paul Apr 30 '18 at 12:37
  • Are there any other codes @Paul? – GdD Apr 30 '18 at 13:09
  • No other codes are present. – Paul Apr 30 '18 at 13:26
  • That's an odd issue, you're saying that the 0140 code only comes up when you're driving the car, never at idle? – GdD Apr 30 '18 at 13:31
  • Very odd. I'm averaging about 35 mpg which is above the original specs of the car and like I said, the upstream sensor is reading within specs. – Paul May 1 '18 at 21:28

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