oxygen sensors: front or both

[JHZR2]

Hi,

My mother's 97 plymouth breeze has stated that it wants a new O2 sensor.

The car gave me the codes by switching it on and off a certain way, and counting blinks.

The code referred to oxygen sensor malfunction or voltage not fluctuating.

My issue is, there are two: one pre cat and one post cat. From what Ive researched, the post cat one is more or less useless, and only there to verify that the converter is working.

I will change the front sensor. However, I have a bit of logic and wonder if I am on the right track:

If the rear sensor verifies catalyst operation, if it was the bad sensor, I would get the oxygen sensor code, AND a code relating to a malfunctioning catalytic converter. Or, I might only get a code relating to a bad converter.

On this logic, I assume that it is only the front sensor. Make sense?

Of course it is easy enough to swap the main one, and if the light continues, to then replace the rear one at a later time. But it would be nice to have an idea, and based upon this idea, potentially save an extra trip to the parts counter...

So, what is yout thought? Front or both sensors? I dont mind spending the extra $100 if its best for the car, but I believe that it may be a useless swap unless it isnt working, since it does not apparently effect anything.

Thanks,

JMH

 

[55Test]

I think you're on the right track. If the rear sensor on an OBD2 car is bad, then there would be a stored code for it, then a code for reduced catalyst effiency, or some other wording regarding the catalytic converter.

 

[nel]

Best to replace the front one because it affects engine operation. If you still have a problem, then use the old front one for the rear. The rear one is pretty useless--it's there just to verify that your emissions equipment is working properly (in compliance with government mandated OBD-II).

The old O2 sensors used to only last 35K-50K. Nowadays, the technology has improved so that the sensors can go 75K-100K. When the sensor starts slowing down, the engine control system doesn't correct the air-fuel mixture as well during transients (e.g., throttle-in & -out manueuvers). Also, during extended idles, you can get into a situation where the O2 sensor goes to "sleep" because it's cooled down, and then your idle can become erratic.

 

[robbobster]

quote:

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Originally posted by nel:
Best to replace the front one because it affects engine operation. If you still have a problem, then use the old front one for the rear.

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If you go this route then make sure that the rear sensor is the same part as the front. At least on my truck, the rear sensors have different part numbers.

Though I am unsure if they differ in operation, I would assume so given the different part numbers. Perhaps the voltage they send to your ECU would be different, I just don't know for sure.

 

[cryptokid]

its normally the stupid connector or the length of wires which give the front and rear sensors different part numbers.

the sensors themselves are almost awalys 100% identical.

 

[Kestas]

Mr. 55 is on the right track. I looked at the list of available codes using the key-off-on technique.

Code 21: Oxygen sensor signal doesn't change (stays at 4.3-4.5V). Probably bad oxygen sensor

Code 64: Catalytic converter efficiency failure

I suspect only the front O2 sensor needs replacing.

Isn't $100 kind of steep for a sensor? I would think that if you shop around you should get it for $35.

 

[mechtech]

Nobody want's to pay more, of course.
But I'll gladly pay for quality, in this critical piece of engine management.