To test whether either or both O2 sensors are damaged, one hooks the (black, following Bosch's convention) wire on each sensor and tracks the signal against the ground using an oscilloscope.

When the car is idling at 3000rpm, the upstream sensor will show a wave with an amplitude in the range between 0.2 V and 0.8 V, and a frequency between 1 Hz and 2 Hz.

The signal looks like this:

pre-cat signal

The downstream sensor should have a much more constant signal. This stable signal indicates that the catalytic converter is doing its job and there is no fluctuation in the level of O2, or not as much, past the cat.

How can an engine revving at 3000 rpm produce a variation in O2 of frequency 1-2 Hz rather than 50 Hz? I'd have been surprised if the O2 variation was anywhere near 3000 Hz; there surely must be enough turbulence in the engine outflow for any variation to vanish, but I'm still surprised that there is a variation at this low frequency. What causes it?

  • @Moab - Make your mark, brother ... please turn this into an answer. Commented May 14, 2016 at 13:12

1 Answer 1


Upstream Variation (Hz) is also called O2 crosscounts, this normal fuel mixture control, the fuel computer changes mixture ever so slightly so it crosses the stoichiometric boundary (.45v), back and forth from lean to rich. The more crosscounts the better fuel control is and indication of a healthy O2 sensor, high crosscounts is also an indication the fuel computer is in closed loop mode.

More reading on this subject

  • So by way of analogy we could say this. Just like someone trying to walk on a rope moves ever so slightly from right to left to remain in balance, the computer changes the mixture from lean to rich in an attempt to continuously find that perfect stoichiometric point. It is this change between rich and lean that has a frequency of 1-2 Hz. Is that right?
    – Calaf
    Commented May 14, 2016 at 17:00
  • Hz is how many times in a given amount of time it crosses back and forth, the faster it switches the higher the Hz.
    – Moab
    Commented May 14, 2016 at 17:27
  • As an example, AC current in the USA is 50-60Hz, it switches back an forth at 50-60 cycles per second.
    – Moab
    Commented May 14, 2016 at 17:30
  • Yes, I understand. Your explanation clarifies why we observe 1-2 Hz, but not why the ECM is changing the mixture. I'm still wondering why the on-board computer moves the mixture from rich to lean continuously. I'm guessing it's because it's very difficult to hit the stoichiometric point exactly head on. And so the ECM hovers back and forth in an effort to locate it. This is easier than if the ECM would, for example, analyze the exact composition of the fuel at each moment and the exact amount of salts etc in the air, and then compute from the analysis what air-fuel ratio it should use.
    – Calaf
    Commented May 14, 2016 at 17:32
  • 1
    Remember that computers see on or off, reading exact voltage is harder/more expensive. If the ECU reads rich/lean its easier. rich = 1, lean = 0. The ECU constantly cycles to see the switch happen and know that is is around stoichiometric.
    – rpmerf
    Commented May 16, 2016 at 10:44

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