I have very slow primary (pre-cat) oxygen sensor readings on my 02' Corolla (2ZZ-GE, 141kW N/A).

While trying to investigate the issue, I repeated diagnostics frequently. Before every diagnostic attempt, I fully warmed the engine and raced it @ 2500 RPM for about two minutes.

Output of oxygen sensor readings:

  • typical 0.1Hz, max. 0.2Hz @ idle (very slow and not constant)
  • typical 0.5Hz, max. 0.8Hz @ 2500 RPM (extremely slow and not constant)
  • Graph


Additional and possibly related troubles:

  • Erratic idle
  • Engine speed follows oxygen sensor signal
  • LTFT -15% at @ idle and about 0% on next minimal TPS reading

I have tried the following:

  • changed to a new OEM oxygen sensor (primary, pre-cat).
  • changed to a new OEM MAF sensor.
  • changed to a new OEM PCV valve.
  • changed to a new set of spark plugs.
  • changed to a set of used, low millage and cleaned OEM fuel injectors.
  • cleaned throttle body and idle air control valve.
  • checked VVTL-i (lift) valve and filter. Working and perfectly clean.
  • checked for intake air leaks. No were found.
  • checked for exhaust air leaks. No audible/evident leaks were found.
  • reset ECU.

No success at all. What else can I try? Thanks :)

  • Which O2 sensors are you looking at? Pre or post cat? Post cat (typically called sensor 2) will only show a slow roll. The post cat sensor has nothing to do with how the engine runs, but rather just monitors the efficiency of the cat. Commented Oct 11, 2016 at 13:16
  • I'm looking at primary (pre-cat) oxygen sensor. I just added this to my question, thanks!
    – Fanaz
    Commented Oct 11, 2016 at 13:19
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    If you are using Torque Pro, you can post up pics of what you're seeing. Please do so. I'm not doubting your abilities, but we all make mistakes. Sometimes we cannot see the forest for the trees. When I say "we", it's all inclusive to include myself. Just trying to make sure we're all on the same footing here and talking about the same thing. Commented Oct 11, 2016 at 17:16
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    @Fanaz Note that those year corollas had an ECU recall for recalibration. Call a dealer and see if it was done.
    – race fever
    Commented Oct 12, 2016 at 14:05
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    @FredWilson - Do you have documentation to prove either point? Everything I've ever read states the post-cat O2 is used to judge cat efficiency. If the cat is doing its job efficiently, there's nothing there to adjust for, so not sure how it could be used for anything but. I'm willing to believe what you're saying, but not blindly. Show me where I've gone wrong. Commented Oct 12, 2016 at 15:56

3 Answers 3


symptoms: LTFT -15% at @ idle and about 0% on next minimal TPS reading.

What is going on: your injection ecu is reading rich condition and reducing fuel injected when fuel system is in closed loop


    1. Makes me thinks of a tiny leak when injector not closing well/cloged, injector leaking a tiny amount of fuel => when rev up RPM leak amount become proportionally small when high amount injected.
    1. Or fuel pump/feeder pressure outputs too high and injector are fine. and RPM at idle is smooth.
    1. Unlikely but fuel octane in tank is too high for the engine ?
    1. air intake manifold sensor lazy


  • the proper clearance of Crankshaft position sensor

  • And if any camshaft position sensor. And clean them up. Those can have magnet and they collect metallic debris.

  • ECU: recheck ECU wire socket for dirt oil rust. and harness you will probably check resistance with multi-meter and read 0-1 ohm

uneven idle/ rough/ Shaking engine/ trouble with firing or EOBD or scope : try to plot the engine speed at idle with high resolution coming from the Crankshaft position sensor if your engine is a 4 cylinders: (couple of cylinder firing/spark ignition) you should see a pair a bump in speed (two waves like when each cylinder fires) you have 3 cylinders that are starving fuel from the leaky cylinder. your speed bump wave will increase differently one small wave and another bigger

when cylinder is rich typical it produce more tork than others hence biger rpm jump when if fires


You stated the O2 in Hz. Typically the O2 moves below and above.5. So, when I am diagnosing I will be looking for it to cross the .5 volt threshold about every 5 seconds. The ECM doe compare post and pre O2's to calculate converter efficiency. I see you replaced many parts;but, not the O2?

  • As I figured out, the reason the OP is stating in Hz is for the frequency of the state change (how often) from lean to rich, not the output value of the O2 sensor. Commented Oct 16, 2016 at 12:27

Assuming these are narrowband O2 sensors, the sensors should cycle far quicker than that once they are up to operating temp.

From experience, an ageing O2 sensor will go lazy with time and exhibit the sort of behavior you're observing. In all probability you will have to replace the pre-cat sensors.

This video provides a useful reference of functional vs lazy sensors.

  • As mentioned previously in original post, I have changed to new OEM pre-cat sensor. Didn't help at all. Where should I look now? Maybe valve clearance?
    – Fanaz
    Commented Dec 21, 2016 at 6:57
  • @Fanaz no this sounds like a control issue, not a mechanical one. Are you sure the wiring for pre and post isn't switched around? youtu.be/WrCa9cC9EPg
    – Zaid
    Commented Dec 21, 2016 at 7:00
  • Yes, I'm sure wiring is ok. The list of additional problems like erratic idle and very low LTFT at idle makes me think it could be mechanical. Could these symptoms be caused by unadjusted valves or VVLT-i failure? Another thing is throttle response. At idle TPS shows 11.8%, which is within spec. When I carefully press throttle pedal for a few milimeters reaching next TPS reading "step" (~12.1%), engine speed instantly rises from 750 to about 1600 RPM and LTFT jumps from -15% to 0%. I have a feeling that all these things are related to each other, but still can't find - how.
    – Fanaz
    Commented Dec 21, 2016 at 7:33

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