o2 sensor replacemet: philosophy or science?

Old 02-14-05, 10:48 AM
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o2 sensor replacemet: philosophy or science?

The parts stores seem to be pushing sales of O2 sensors much like air fresheners. Is there value in changing the O2 sensor only because a certain number of miles have elapsed? Is it not true that the car’s computer will say when a sensor is no longer functioning?

I looked at the Bosch web site for recommendations for cars that I have owned drove for years and sold while mechanically perfect and with legitimate emissions stickers, and on which I never changed the O2 sensor. The cars were a ’84 Pontiac 6000, 2.8L-V6 with variable jet carburetor and ’84 VW Vanagon, 1.8L flat-4 water-cooled w/ FI. I do not claim that the cars were trouble-free just that I never thought about 02 sensors and never had problems passing emissions tests. The Pontiac I got w/ 60K and sold w/ 124K. The VW I bought w/ 160K and unloaded at 220K. On the VW, a previous owner had disconnected a counter on the speedo cable that was to light up an O2 sensor light on the dash. I do not know what the interval should have been or if the light was to mean, “replace” or “have it checked.” I discovered the box while replacing the speedo cable. I have no similar memory of the Pontiac.

For both above cars the Bosch web site says replace at 30K intervals. What do you think? I know this puts money in their pocket. I do not own those cars, but I do own cars w/ O2 sensors that the site recommends replacing at 50 and 100K intervals. The cars have gone 40 and 54K beyond that.

I have read and heard about “lazy” and “slow” sensors. Is there such a thing? The car manufactures could easily program a diagnostic that would make the car to run too rich, then too lean for a second, and see if the sensor picks it up. This would eliminate the guesswork.

I am thinking about this now, because one car is up for re-inspection this spring. Last year it passed, but HC was high. I remember it being about 220 ppm with the cut-off being 240 ppm. In the car’s youth, I remember the HC being in the single digits. Changing, the sensor on this car will take about 2 minutes, since it is very accessible. Is it worth 2 minutes of labor, $50 for the part and 30 minutes of waiting at the parts store for 3 managers to sign-off on giving me my deposit back on the “free” loaner tool. Because this is so easy, it seems like a very attractive repair for anyone w/ $50 to spend on his or her car.

Thanks for your input.
Old 02-14-05, 04:52 PM
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In the 20 odd years since O2 sensors became commonplace on North American cars, I have only seen a handful that required replacement.

If we keep our cars in decent condition and states of tune, don't use suspect or bargain gas (gasoline is seasonally blended - some bargain outlets will buy a refineries past dated overproduction for pennies on the dollar and flog it for slightly less than "good" gas) and don't introduce anything into the motor bearing silicone, I expect to see many, many miles of service from an O2 sensor.

About the only two real problems that we will see with an O2 sensor are problems with the heater circuit or (due to allowing those things we shouldn't do) the sensor becomes contaminated or "lazy".

In all those years, I have yet to need an O2 sensor in any of my own vehicles.
Old 02-14-05, 05:12 PM
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I seem to have run into a few more than Jim has. I had one today that was used up. Sensors do deteriorate with time, mostly because of geting contaminated. An O/2 sensor is a voltage generator. What changes is the amount of time and heat it takes to "wake it up" and how sensative it is to small changes. The reason we don't change them as often as we should is because we have such good computers that have the ability to adapt to changing readings coming from sensors. The problem is the computers are responding with more fuel because the sensor isn't reading as high as it should have been, thus reducing fuel mileage. Once the computer has reached it's adaptive limits and no longer can compensate, it will signal that the sensor is not keeping up. Many newer sensors are equipped with heaters built in to bring them online faster. That's something else to go wrong with a sensor. Driving a car with a bad head gasket for any period of time will also destroy a sensor. The bottom line is, there are benefits to refreshing sensors after they have been used 100K plus or if any of the previous events damage it.

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