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Oxygen sensor negative output

Discussion in 'Electronic Design' started by Adrian Tuddenham, Nov 25, 2012.

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  1. I have been testing a Bosch automotive exhaust gas oxygen sensor of the
    zirconium dioxide type, which I suspect is faulty. There are four
    connections to it: two wires to a floating heater element, which require
    12v of no particular polarity; one sensor output wire and one sensor
    earthing connection to the body of the device.

    The heater correctly takes about 1.7A when cold, falling below 1A as it
    warms up. The sensor is supposed to give readings between +0.2V and
    +0.8v when exposed to exhaust gases, the voltage becoming more positive
    with decreasing oxygen level. Instead, it gives readings between -0.8v
    and -0.2v, with the voltage becoming more positive with decreasing
    oxygen levels. In other words, the readings are behaving correctly but
    are displaced about 1.0v negative.

    According to various websites, this is typical behaviour for a sensor
    which has become contaminated with silicone residue, and the obvious
    solution is to replace it. However, I can find no source for such a
    contamination and some websites imply that the contamination might be on
    the reference surface of the zirconium oxide 'thimble', not on the side
    exposed to the exhaust gasses.

    My question is: does anyone know which side of the sensor is the
    contaminated one when the voltage is displaced negative?

    I realise this is not exactly a 'design' problem, but I think that I
    shall only get a proper answer, as opposed to hearsay and guesswork, by
    asking designers who have actually worked with such devices.



    [I have already eliminated bad earthing contact between the exhaust
    system and the engine, which is the commonest cause of a spurious
    voltage readings. The vehicle is not consuming large quantities of
    coolant, so anti-freeze contamination is not very likely. To the best
    of my knowledge no repairs have been done with silicone sealants]
     
  2. Guest

    The theoretical output voltage is:
    Cell output = (2.303RT/4F)x log ( P1/P2)

    where:-
    R = molar gas constant
    T = absolute temperature of cell in Kelvin
    F = Faraday constant
    P1 = partial pressure of oxygen in the reference gas
    (air in most cases)
    P2 = partial pressure of oxygen in the sample

    So negative voltages imply the partial pressure of oxygen in the exhaust stream exceeds that of the air at the sensor reference port.
     
  3. miso

    miso Guest

    Just a FYI. If this problem is causing too rich of a mixture, just
    replace the parts before they ruin the catalytic converter. I've seen
    this in defective mass air flow sensors. i don't know how this O2 defect
    will effect the fuel mixture. [Well, I rather not venture a guess since
    I'd hate to be the person who said it caused the mix to be lean, which
    is just a power issue, versus too rich which kills the catalytic converter.]

    Basically raw fuel makes the catalytic converter run very hot.
     
  4. I had wondered if the reversed reading was caused by an exhaust leak
    causing gasses to stream past the reference port of the sensor, but
    there is no sign of a leak near of the sensor. (There is a slight leak a
    few feet away in the back box.)

    The negative reading continues with the engine switched off and the
    sensor heater sustained by an external battery, so it seems as though
    either the sensor port is blocked and contaminated or the sensor element
    is poisoned. As the sensor has only been in use about 18 months, I
    suspect the latter, but I will try flushing out the port with clean
    Iso-propanol as a precaution.
     
  5. The mixture has been all over the place, with prolonged bouts of power
    loss interspersed with intermittent surges. This only occurred after
    driving for a mile or two, which is what made me suspect that it was
    something to do with the fuel control system switching to 'closed-loop'
    operation. Surprisingly there has been no visible smoke in the exhaust
    gasses.

    On occasions the exhaust gasses have seemed exceptionally hot, as far as
    I could tell by placing my hand in the gas flow behind the tailpipe. On
    other occasions they have seemed normal. The hottest (almost at flame
    heat) occurred during idling immediately after a severe bout of
    misfiring, so I suspect that some damage may have already taken place.

    I can only estimate the temperature during idling, as my simple
    temperature testing method obviously cannot be used whilst driving.
     
  6. Guest

    From what I can gather from online diagnostics manuals, most fuel controllers are aiming for +0.45V average out of the O2S, with the output swinging from +0.2 to 0.8V somewhere in the range of 0.5-5Hz rate. Excursions into negative output voltages are okay as long as the time average comes to +0.45V.. And you're right, it is the excessively hot exhaust that damages the converter.
     
  7. miso

    miso Guest

    You really need the shop manuals for voltages. I have 4 O2 sensors, and
    not all have the same voltage range.

    Frying the catalytic converter is far more expensive than taking it to a
    garage. This is one of those no win situations, where you pick the
    lesser of two evils. Kind of like voting in a Republican primary. ;-)

    This reminds me of the old radio show Car Talk, where the callers had
    put black tape over the check engine light for a year before getting
    around to fixing the problem.
     
  8. There are two basic types: a voltage-generating system base on Zirconium
    and a variable resistance system based on Titanium. I have managed to
    establish that this is a Zirconium-based sensor which means that the
    voltages are determined entirely by the electrochemistry of the process
    (see other replies to this thread).

    Some vehicles have multiple sensors at different places in the exhaust
    system and I suspect that their readings would differ according to what
    they were sensing, which would account for your observations. However,
    no Zirconium-based sensors are supposed to give negative readings, no
    matter what the exhaust conditions, which is what mine has been doing.

    Luckily my vehicle was made in a year where a catalytic convertor was
    not mandatory and the emissions limits fairly lax. As long as it passes
    the appropriate emisssions test for a vehicle of that age, I may be able
    to get away without a functioning convertor.

    There would be a lot less of that sort of thing if the manufacturers
    provided proper diagnostics which were easy to use without specialist
    equipment. When pintable [pinball] machines first started using
    microprocessors, there was a simple built-in diagnostic system which
    allowed the mechanic to read the signal from every sensor and to
    test-operate every solenoid. With the elaborate dashboard displays in
    modern cars, a similar system could be added for the cost of a couple of
    switches and a bit of software.
     

  9. Many thanks for the advice. I think I have now tracked down most of the
    causes of the problem. The sequence of events seems to have been:

    1) The fuel filter was choked, also the pump may have been worn out.
    This meant that, on heavy fuel demand such as climbing a hill, the
    pressure at the injectors fell.

    2) The engine management system compensated for the reduced fuel
    pressure by lengthening the injection period. When the throttle was
    shut at the top of the hill, the fuel pressure rose quickly and the
    engine flooded.

    3) The extra fuel burned off in the exhaust system and damaged the
    oxygen sensor. Also, the engine management system either went off-scale
    or got in a tangle, which caused it to lose track of what was really
    going on.

    Replacing the fuel filter, fuel pump and oxygen sensor has put things
    right but revealed a trivial underlying fault, which must have been
    confusing the situation even more: the throttle stop switch was oily and
    only made intermittent contact.

    Once again, thanks to all contributors on this and other threads for
    helping to save the vehicle from the scrapyard (for now, at least).
     
  10. Guest

    does it even measure fuel pressure? didn't think many petrol cars did
    they just have a mechanical pressure regulator recirculating fuel to
    the
    tank

    and I don't think it would explain flooding, when you shut the
    trottle
    the injector period go to zero almost instantly, low or high fuel
    pressure
    the engine would floor with full on injectors and no air
    Generally the ECU won't adjust more than a limited percentage based
    on
    the sensor, it can run reasonably in open loop

    sometime sensors just go bad, wear out or what ever, every second year
    or so
    one of the two on my car needs replacement
    -Lasse
     
  11. It does have a mechanical pressure regulator. Perhaps I didn't explain
    myself very well. I think the fuel pressure drop during acceleration
    caused a weak mixture which the oxygen sensor picked up. The ECU then
    attempted to compensate for that by lengthening the injector period.

    The engine was cutting out and giving intermittent low power, but the
    inertia of the car kept it turning. The faulty throttle-stop switch
    meant that the air by-pass valve was fully open a lot of the time
    anyway, so there was always some air coming through. Once the injectors
    reacted and stopped flooding, the engine would return to a fast idling
    condition which allowed me time to change gear and to keep the vehicle
    moving.

    Drivers behind me were often annoyed.

    It ran much better open-loop when I eventually disconnected the oxygen
    sensor altogether.
    I will keep an eye on things in future, and treat even
    recently-replaced items with suspicion.
     
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