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rear or downstream O2 sensor

edited November -1 in Repair and Maintenance
OK, the heating circuit has failed on my downstream O2 sensor(CEL code P0141). As I understand it, this sensor does not affect fuel trim but only monitors exhaust. My Subaru still runs fine with a very slight decrease in MPG. Dealer wants over $300 to replace and says if the sensor is not replaced, damage will occur to catalytic converters. If this is true, please help me understand what is happening. Seems as though the downstream sensor has minimal to no affect on engine performance. Definitely not running in limp mode.
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Comments

  • edited April 2009
    the rear o2 sensor makes sure that your cat is still doing it job of burning any on burn gas. the dealer is right it can damage your cat but most likly not, but if it the check engine light flashes you need to turn off the car and bring it to the shop becuse that means that it could damage tha cat.
  • edited May 2009
    DTC code P0141 "O2 heater circuit. Bank 1. Sensor 2" does not even describe what is wrong with the heater circuit. It simply says that there is a problem with the heater [b]circuit[/b]; that's all. It doesn't say that there is a failure inside the O2 sensor. If this code is the only thing the dealer has, then, it's a wild jump to the conclusion that the O2 sensor has failed.
    Kuiama, you have the wrong repairer. Your repairer doesn't have s/his facts straight on the other statements, either.
    The problem with the O2 heater circuit could be with the supply voltage to it, or, somewhere else.
    [b]DTCs (Diagnostic Trouble Codes) do NOT refer to parts, or components. DTCs refer to [u]voltage values[/u]. Yes, voltage values in [u]component circuits[/u].[/b]
    Billions of dollars a year are spent to replace good parts because mechanics and managers aren't making the distinction between a voltage value and a component.
    If the repairer replaces the O2 sensor, based [i]solely[/i] on the trouble code, that will negate everything I say, won't it? Or will it show that there is such a thing as blind luck?
  • edited May 2009
    With this code set you will be running in open loop regardless of the reason the code is set. Constant open loop operation can't be good for the cat.
  • edited May 2009
    If this code only applies to the downstream O2 sensor then it will not affect the open/closed loop operation of the engine. The downstream O2 sensor controls the heat of the catalytic converter. It slightly richens the mixture to make the cat heat up faster, and it leans it a bit if the cat gets too hot. Basically it keeps the cat in the optimum temp range to work right. It is true that it could damage the cat by making it get too hot if it's not sensing correctly. BUT, the rear O2 sensor will not cause the car to remain stuck in open loop. The front sensor does that.

    Presonally, I'd fix it. It can in fact damage the cat which would certainly cost more than $300 to fix. And while helokit is correct that it might not be the sensor, the sensor would be the most common failure of this type. The heating elements in rear heated O2 sensors do go out pretty frequently.
  • edited May 2009
    Thanks to all who replied.
    Hellokit, I agree with your assessment. I am jumping to a conclusion regarding the condition of the sensor itself. I only checked the outer condition of the wires for breaks in the insulation for possible short to the frame. I haven't yet done any voltage or resistance tests. And the dealer has not looked at the car, I simply asked them over the phone what it would cost to replace the sensor. As JLeather states, the odds are better that the sensor itself has gone bad. I didn't think the car was stuck in open loop...it runs fine with only a very slight decrease (maybe 10%) in fuel efficiency.
    I suppose eventually I will have to replace the sensor despite the high price, the CEL is driving me crazy and probably will not pass inspection with the light illuminated.
  • edited May 2009
    DTC PO141, When the HO2S temperature measures below 360C the sensor will not produce any voltage and will behave like an open circuit. This will result in an Open Loop operation. From the FSM. Closed loop operation requires feedback from sensor 2 not just the front sensor.
  • edited May 2009
    You can check for voltage [i]to[/i] the oxygen sensor heater, and [i]from[/i] the oxygen sensor, with an electrical multimeter. You could check the resistance of the oxygen sensor heater, also.
    To see if the PCM (engine computer) is making a ground for the oxygen sensor heater circuit, you would check for a voltage drop, when the ignition switch is turned ON for a few seconds, on the heater ground wire before it goes into the PCM.
    Don't bet on the dealer's mechanics performing these simple voltage checks.
  • If I understand your question correctly you want to know if it's really necessary to replace the sensor and if so, an explanation as to why.

    The downstream sensor only monitors the performance of the cat converter by providing a second signal to the computer to compare with the upstream sensor (which DOES affect trim) and see if the converter is working properly.

    Not fixing the probem won't affect the fuel trim or the converter. However, it effectively eliminates the function of the CEL. If it's always on, how will you know if a pending engine-destroying disaster is about to happen?
  • edited May 2009
    Thanks Hellokit...I do have a multimeter and I will perform these tests before I condemn the sensor. I have a funny feeling the sensor itself is shot...I can purchase the proper sensor for under $100 and would probably save myself $200 by replacing it myself, and although the thing is rusted in there pretty good, at least part of the heat shield is broken and loose, so I may have a good chance of getting a wrench on it.
  • edited May 2009
    Indeed, mountainbike, it's the fact that the CEL is on all the time that worries me, although if I reset the ECU by disconnecting the battery, the computer allows me one full driving cycle before the CEL illuminates again. I just have to keep disconnecting the battery every time I turn over the ignition, then I never see the CEL!
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