To test the upstream O2 sensor on a 2011 Toyota Corolla, I backprobed it with a multimeter.

After I idled the engine for a few minutes to warm it up, the multimeter shows a steady voltage of 0.4V.

But I thought O2 sensors were supposed to fluctuate between 0.1 - 0.9V, so it looks like something is wrong.

After revving the engine to 2500rpm, the multimeter still shows a steady voltage of 0.4V.

What could this mean?

  • 1
    What is the voltage at start?
    – GdD
    Commented Mar 30, 2021 at 16:53
  • @GdD Right after starting (and before warming up), it was also 0.4V. It stayed there after warming, too.
    – Asker
    Commented Mar 30, 2021 at 18:00

1 Answer 1


I am thinking you are measuring reference voltage to an A\F O2 sensor. Reference voltage is usually between 2.5 and 4 volts. If there are 4 or more wires, this is most likely the case. Also A\F sensors don't oscillate like traditional sensors.

  • There are indeed 4 wires, so you might be right. I just ran an OBDII scanner, and it shows a steady 3.3V for this sensor ("O2B1S1"), which also fits what you said. My voltmeter, though, shows 0.4V. What could explain this difference?
    – Asker
    Commented Mar 30, 2021 at 18:02
  • Reference voltage is altered when a rich or lean voltage is detected. When your scanner or sensing voltage match, you have a perfect mixture. If sensing voltage is less than reference it is a rich condition. If higher it is lean condition. I don't know what the .4 volts is. I don't know what wires you are probing. If you got a pin out diagram you can then figure which signal to check for each wire.
    – Jupiter
    Commented Mar 30, 2021 at 19:05
  • If I'm understanding correctly, then the reference voltage is the range 2.5-4V, and my air-fuel mixture is balanced if the voltage shown by my scanner is in that range. Is that correct?
    – Asker
    Commented Mar 30, 2021 at 19:13
  • Also, I am backprobing the positive and negative signal wires (yellow and blue) on the ECU-side connector (female connector) that my O2 sensor plugs into
    – Asker
    Commented Mar 30, 2021 at 19:14
  • Maybe you are complicating things a little much. Probing gets a little complicated on these sensors. If you are using a scanner, this is sufficient. I assume you have no codes. If that's the case you are good to go. For a good education on these sensors you can Google A\F O2 sensors or Lamda O2 sensors. There's some really good articles.
    – Jupiter
    Commented Mar 30, 2021 at 21:24

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