I was checking my short term fuel trim numbers and o2 sensor switching speed as I think my post-cat sensor has degraded.

The car is not throwing a code, and I don't believe that there's a hole in the exhaust but I am open to correction.

At both idle and 2500 RPM, after engine warm up, the post-cat short fuel trim is hovering about 50%. The post-cat o2 sensor switches with a lot less of a voltage difference than the pre-cat. The STFT pre-cat drops to about 1% at engine warmup.

I estimate the lambda to be about 1.75, which is far far too high.

So, is the post-cat lambda dead?

  • Is the "1.75" measurement in volts dc? If not, what measurement? Also, the post cat O2 should be almost rock steady if the cat is functioning correctly. If it's switching at all, it's likely the cat isn't doing it's job. Commented Aug 14, 2016 at 15:58
  • the 1.75 is the difference between the air to fuel mixture. An ideal lambda reading is 1.00, however you're allowed some variance in emissions testing of ± 0.03. When you say the "the post cat O2 should be almost rock steady" do you mean voltage or STFT?
    – Nick
    Commented Aug 14, 2016 at 16:04
  • What kind of vehicle are you running this on? What engine? How are you figuring pre/post cat? The post cat shouldn't have anything to do with the fuel trims. I'm wondering if you're looking at bank 2 STFT and thinking this is post cat? This would be for a V-engined vehicle. All the secondary O2 sensor does is check the operation of the cat, nothing else. This is why your question is confusing me. Commented Aug 14, 2016 at 16:09
  • By "secondary" I mean to say "post-cat". Commented Aug 14, 2016 at 16:15
  • Ahh, I assumed bank 2 STFT was post-cat. 4 cylinder 660cc engine - so only a single pre and post cat o2 sensor. My data is from a dealers scan tool.
    – Nick
    Commented Aug 14, 2016 at 16:22

1 Answer 1


The post o2 generally does not effect performance. It's rather more a gauge of how well your catalytic converter is functioning. In a perfect world Stoichiometry is what the secondary is trying to listen for. The reason for switching of your pre-cat o2 sensor is to allow for temperature and fuel/air fluctuation to efficiently allow the cat to absorb both the lower and higher spectrum of pollutants that the engine excretes during combustion. You will generally see a more steady or consistent line of data on the post cat o2 simply because it's providing efficient cleaning of your emissions. 50 percent or close to it isn't necessarily a bad number as long as your engine is designed to run that way.

Another thing to consider is the scantool that they used to scan the vehicle. I've seen technicians use in house tools, and after-market tools that don't always work. I have several tools that cost in the thousands and sometimes they don't read fuel trim and o2 data correctly. You may need to make sure they are using an in house scan tool made by the engineers that developed the computer system for your car.

If you believe the results are too high, you can use a good multi-meter or oscilloscope to compare fuel injection pulse to your downstream o2. o2 sensors are a bit tricky. I'm not a "parts changer", but most times when I run into an issue like this that points towards a bad o2 sensor; I replace it and go from there. You can always clear your ECU/ECM and install it. See if it works and if not, just take it back to the parts store and go from there. Look for bad wiring and warping of the o2 as well. They take a real beating sometimes because of the temperature fluctuation.

Also, as far as throwing a code. They really need to be high on the STFT and LTFT to throw a code. Generally over -15% or +15% will throw a code. It's a bit silly that it takes that much, but you also didn't describe the running condition of your engine. Usually when you have a very bad fuel trim, the vehicle will run like crap.

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