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I noticed recently that my gas mileage on my 99 Nissam Almera 1.6L ( EU version of the Sentra basically ) has suddenly gotten really horrible. I connected to the Nissan Consult interface and used the Consultz software to view some ECU data, and at idle it was showing 0.05v, while running at 2krpm it was oscillating fairly quickly. The car is not idling high or rough, which leads me to believe it's not an air leak. It uses a heated 3 wire zirconia HO2S. The resistance for the heater element was just barely in spec at 4.3 Ohms. There was B+ on the heater power pin. I hooked up my UT61E multimeter which is a 22k count TrueRMS and it showed the signal bouncing between about 0.03v to 0.1v. So far it looks like a bad O2, however I decided to hook up my Hantek 1008C to look at the signal and this is what I got at idle:

enter image description here

and this is what I got at 2krpms:

enter image description here

The thing that confused me was that I expected to see a signal oscillating and not this crazy mess. Basically, I'm pretty sure it's the Lambda that's bad, but those waveforms really surprised me.

Is the Lambda bad? Are those waveforms real or do I maybe have a defective scope?

EDIT

The scope has a 1Kh 2 volt signal generator, and I hooked up to that signal and it looks fine:

enter image description here

So I'm guessing the scope is OK and that is just one really messed up Lambda?

Updated Data

OK, so I was doing something wrong I guess before, because now I was able to get a good scope of the Lambda sensor. I recorded three 60 second videos, one of the scan tool data, one of the Lambda scope and one of the four cylinders' sparks.

Basically, the idle is completely normal, both in RPMs and smoothness, yet the O2 is pegged at about 0.05v. When I raise the RPMs to 2500, it starts switching, and when I let off it hangs high, switch maybe 2 or 3 more times then goes back to being pegged low.

I also compared the sparks. This model has a dizy with a built in CMP sensor and coil. I'm not sure exactly what these are supposed to look like, but one thing stood out to me, the Cylinder 1 spark was only about 0.8ms, compared to 1ms on all the other cylinders, and seemed to have a firing line about 1 or 2Kv higher than the other cylinders. Could be related, or completely separate issue.

The car usually gets about 10+ k/l, but it's now getting about 6.7 k/l.

Update Nov. 9th 2017

So here is what the signal looked like before at 2500 RPM:

enter image description here

It's taking over 200ms for it to switch from rich to lean.

And here is with the new Lambda sensor, also at about 2500 RPM:

enter image description here

The frequency is about twice as fast, and the switches are only taking about 100ms now. I'm not sure why it's all wobbly like that, maybe be my cheap scope.

I'll have to drive it around for a few days and see what the results are, but it seems like the old O2 was lazy, so I'm going to accept Ben's answer and if there is still a problem, I'll open a new question.

  • The obvious thing to do is to connect your scope to a known clean signal then you know if the output is cr"p the scope is cr"p.... – Solar Mike Nov 1 '17 at 11:37
  • @SolarMike I guess. I connected it directly to the battery and it showed correctly 13.8 to 14 volts. It has a built in signal generator, maybe I'll try that real quick. Regardless, am I correct in assuming it's probably a bad Lambda? – Robert S. Barnes Nov 1 '17 at 11:40
  • if you can't be sure of the data - how can you be sure of your diagnosis? – Solar Mike Nov 1 '17 at 11:49
  • @SolarMike Well I just hooked it up to the built in 1Kh signal generator and it seemed to read it no problem, I posted a screen shot. – Robert S. Barnes Nov 1 '17 at 12:05
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    How do you define horrible @RobertS.Barnes? What was it before and what is it now? Do you have a difference in power? – GdD Nov 1 '17 at 13:05
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Per my comment the o2 sensor is switching slow. Generally spec is something like 10 crosscounts per 100ms.

The secondary ignition patterns look weird, but it may just be the sample rate and timebase being used.

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