So - I have been called a cheap *** before and I know that I am going to regret making this thead

Is there any point in replacing both o2 sensors when I only have a CEL for the upstream (pre-cat) sensor)?

My reasoning is that I have seen countless times that the rear o2 sensor is basically useless and makes zero impact on efficiency or even engine operation. Is this true, or just nonsense? Both appear to be factory original with ~200K miles on them.

I don't mind spending the $40 for the rear sensor, but if it won't make a difference I would rather just put that $40 towards food/rent/student loans/etc. But at the same time, if it will give me a smoother idle or increased economy, then I see no reason not to replace both of them.

I did pull the code and it is indicating a bad upstream sensor (and has been for a few months).

Upstream; 23151
Downstream; 23099

 

Use ntk sensors. There is no reason to replace the rear sensors unless you have a CEL or are failing emissions tests. It simply monitors the cat.

 

The downstream O2S works with the upstream O2S to set a goal voltage for peak Cat efficiency and fuel economy.


The belief that the downstream O2S only monitors the Cat is a myth. Both O2S' do this.


O2S's have a useful life expectancy of around 150,000 miles.


If you leave an old O2S in with a new O2S you will get a MIL eventually.


Here's some data pertaining to the upstream and downstream O2S, taken from the FSM.




Upstream Sensor:

The upstream O2S sensor is located in the exhaust downpipe before the catalytic convertor. It provides an input voltage to the PCM. The input tells the PCM the oxygen content of the exhaust gas. The PCM uses this information to fine tune fuel delivery to maintain the correct oxygen content at the downstream oxygen sensor. The PCM will change the air/fuel ratio until the upstream sensor inputs a voltage that the PCM has determined will make the downstream sensor output (oxygen content) correct. The upstream oxygen sensor also provides an input to determine catalyst efficiency.

Downstream Sensor:

The downstream heated oxygen sensor is located near the outlet end of the catalytic convertor. The downstream sensor is also used to determine the correct air fuel ratio. As theoxygen content changes at the downstream the PCM calculates how much air fuelratio change is required. The PCM then looks at the upstream oxygen sensor voltage and changes fuel delivery until the upstream sensor voltage changes enough to correct the downstream sensor voltage (oxygen content). The downstream oxygen sensor also provides an input to determine catalyst efficiency.

 

I agree with CCKen completely on this one.

The downstream sensor is important and it does figure into the calculation of things like fuel/air, along with measuring catalytic converter efficiency.

Chase and resolve every code. And yes, use NTK oxygen sensors as replacements.

 

I removed my cat on the 97. I get a CEL full time now. Reads bank 1 fail.

So, yes, downstream is important.

 

Thanks CCKen - that was exactly what I was looking for.

I just needed to confirm that the rear o2 sensor had some impact on ECM parameters.

Rock auto has the cheapest price for NTK sensors that I can find. Pulling the trigger on those tomorrow morning.

 

Don't forget rock auto often kills you on shipping. I ordered mine off amazon.