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Heated Oxygen Sensor


I own a 1998 Ford Explorer with 162,000 miles. I took my Explorer to a dealership early one day because the ?check engine? light was on. After having the car for about eight hours, the dealership identified Sensor 2 in Bank 1 as having failed and said the problem was with a Heated Oxygen Sensor. After they diagnosed the problem, they called me and asked if they could keep the car overnight to run tests. I was unable to comply with this request and took the car home. The dealership wants the car back for at least two days (maybe more) to run tests. Is it reasonable that the dealership should need this much time to evaluate an oxygen sensor?

There is another aspect. The dealership said that the car was running lean. I checked some online resources which indicated when the car was running without the oxygen sensor, it is open-loop and runs rich – not lean ? and that cars typically run rich at start-up. Can an O2 sensor fail in a manner such that it causes the engine to run lean or am I receiving bad information from the dealership?

Actually replacing the sensor does not appear overly difficult. Should I just do it myself? I have a voltmeter but not an oscilloscope, and I am not sure I could confirm the waveform.

I cannot think of any reason why the shop should need to car for two days. I also cannot see wny reason why you need to use the dealership for this. Take the vehicle to a reputable independently owned and operated local shop. You’ll save tons of cash and won’t have thes games played on you.

For the record, they can put the output traces of all the O2 sensors on the scope simultaneously, compare the signals, and see exactly what’s going on.

I suspect that if you leave this vehicle with this dealership for two days you’ll have a $3000 laundry list of “must do” items, many of which are unworthy of your hard earned cash.

A code that indicates a lean condition does not automatically point to an O2 sensor.

For example, if the MAF sensor has a contaminated hot wire it doesn’t detect the actual amount of air that’s entering the engine. The computer see’s this signal. If the O2 sensor then see’s more air compared to MAF sensor signal it can set a too lean code.

A voltmeter can’t react fast enough to measure the voltage crosscounts of an O2 sensor. So you either need a scanner that can monitor the crosscounts of the O2 sensor or a scope.

You are correct that the O2 sensors don’t come into play while the engine is in the open loop mode. Once the engine comes up to operating temperature and goes into the closed loop mode do the O2 sensors control the fuel mixture.


Oxygen sensors have a design life of about 100K miles. They cost $69-$89 each for the ones that fit your Ford. There are four of them, one in front and one in back of each catalytic converter. The front ones (the ones that control the fuel mixture) can be a bear to change.

You can buy a OBD-2 to USB adapter cable on e-Bay for $30-$40 that will let you use your laptop to read the outputs of the entire engine control system including all four o2 sensors, CEL codes, the whole package…