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I had read somewhere that some cars show the ignition signal as interference in the oxygen sensor signal when viewed on a scope. It turns out my 98 Mazda 626 is one of those applications:

enter image description here

The full video is here.

Why does that happen and why doesn't it cause problems for the ECU / PCM?

Bonus Question: What are those occasional negative spikes?

EDIT Dec. 19th 2017

I found the source of my question, Shots in the Dark: Engine Misfire Diagnosis:

With some manufacturers, ignition system interference will be present in an O2 signal. This is not to be confused with O2 sensor hash. Ignition system interference is easily discernable from hash because it will occur with the frequency of each cylinder’s ignition event.

Another source, Measuring a zirconia lambda sensor:

Channel 1 (red) shows the signal from the zirconia lambda sensor and channel 2 (yellow) the duty cycle signal for the regulated heating. The sensor signal is toggling between a low and high signal voltage. This proves that the engine management system is adapting the amount of injected fuel to approach the ideal mixture. The sensor signal contains some interference from the ignition and other components. This interference has no influence on the performance of the engine management system because the ECU has filtered inputs.

  • Where did you read that? – Solar Mike Dec 18 '17 at 18:14
  • @SolarMike Don't remember. Maybe one of Scanner Danner's vids... – Robert S. Barnes Dec 18 '17 at 21:45
  • Where are you taking the measurements? At the sensor connector or at the ECM? Where are you grounding the scope? At the battery? What happens if you use the sensor ground? Is the harness routed in such a way that the B+ wire to the alternator is routed close to the o2 harness? Does the noise show up on other components? What happens if you unplug the alternator? – Ben Dec 18 '17 at 22:34
  • @Ben Back probed the sensor connector, grounded to negative battery post. No interference in other sensor signals. Other stuff I'll have to check. – Robert S. Barnes Dec 19 '17 at 8:30
  • @SolarMike I found the sources... – Robert S. Barnes Dec 19 '17 at 16:16
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As you found the source, it gives the answer you are looking for, which is “the ecu has filtered inputs” which means that the input signal has a function applied which would take the average or mean or rms value of the raw signal over a suitable timeframe.

Edit, as you say they design for cheapness and also for what is sufficient - to control a car engine does not require the processing capability of Deep Blue ...

  • I guess the why part of the question is probably just something along the lines of it was cheaper to design it that way... – Robert S. Barnes Dec 19 '17 at 16:46

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