Upstream O2 sensor stays at 0.4V

Question

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?

Jupiter · Answer

I am thinking you are measuring reference voltage to an AF 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 AF sensors don't oscillate like traditional sensors.

Answered by Jupiter on August 12, 2021

Upstream O2 sensor stays at 0.4V

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