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I just bought a 2004 BMW 325ci as a project and have been trying to fix it up. Scanned it and got a couple of codes which I thought were because of a vacuum leak so I smoke tested it and turned out the DISA valve was leaking air. Fixed that and it got rid of most of the codes but there's still P0160 and P0140 anyone know what might be causing this? Voltage of both stays at exactly .43. It seems unlikely that both downstream O2 sensors would stop working at the same time so I'm guessing its some sort of issue with the wiring maybe? Anyone have any ideas of what I should do before I just try to replace both sensors? Thanks in advance.

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  • Welcome to Motor Vehicle Maintenance & Repair! Oct 15, 2023 at 15:18

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The sensors probably aren't at issue. Both codes indicate there is an issue with a fault in the circuit, meaning there's no signal the ECU can detect coming from them.

This website gives the following to help in diagnostics:

  • As with any oxygen sensor code, I recommend performing a visual check of the oxygen sensor harness to check for any obvious signs of damage. They are under the vehicle and are susceptible to damage by road hazards.
  • Next ... check the terminals of the connector for any signs of corrosion or water intrusion. These will cause the voltage readings to be incorrect and will result in an error code.
  • After ... check the resistance of the oxygen sensor itself and make sure it is within specifications.
  • If the oxygen sensor’s resistance checks out okay, you need to measure the reference voltage from the powertrain control module.
  • If all of these checks turn out okay, it is most likely a faulty oxygen sensor. A quick way to check for this is to monitor the voltage of the sensor with a scan tool and either introduce a vacuum leak or an extra fuel source to the vehicle.
  • This should change the voltage reading to one extreme or the other for the sensor. If there is no change this confirms that the sensor is not reading correctly.
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  • 0.43vdc is probably the output signal from each sensor; post catcon O2 sensors are supposed to output a steady voltage around the value you measured as catcon efficiency, since each one has converted raw exhaust gases to pollution friendly HC, CO and NOx. One Beemer (in a BMW forum) still had P0140 after three sensors were put in. Finally, he reported examining wiring and connections and found a 12v wire damaged. Repaired broken wire and error code went away. I don't know number of wires and if yours are heated, making some sensors at least 5-wires..... I'm presuming 12v to power heater.
    – F Dryer
    Oct 15, 2023 at 18:05
  • Thank you for the help appreciate it. Ill update if I figure out whats wrong.
    – AECH
    Oct 15, 2023 at 20:19

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