Hi,

I have a question regarding replacing O2 sensors on a '96 Buick PA (base). One of the sensors (downstream, after the cat) is shot and need to be replaced. I read somewhere that it is recommended to replace the other sensor (upstream, before the cat) as well, to avoid imbalance of data into the PCM and thus low engine performance. Is that correct?

Has anyone a recommendation?
I am looking at the ACDelco pair (AFS75/AFS123) at about 40 bucks each at RA.

Thomas

The upstream sensor is really only there for the computer to monitor fueling conditions(fuel trims). The downstream sensor is only used to see if the cat converter is working or not.

The only real benefit of changing the upstream sensor is if you have a diagnostic scanner that can reset the learned fuel trim values your ECU has stored. Otherwise, changing the upstream would be moot.

Mike,

thanks for the advice.

So, since the upstream sensor is working it is better left untouched?

What would be the effect of not replacing the downstream sensor?
I understand that it measures the O2 concentration in the exhaust gas so that the PCM is adjusting the injected fuel accordingly. Now, since the sensor doesn't report any value, does the PCM use a fallback program with assumed O2 values? Or does it estimate the values based on the upstream sensor?

Thomas

No, the downstream sensor is only used to see if the cat converter is working. It only reports a possible problem with the cat, that'* it.

If the upstream sensor(one that monitors fuel trims) fails, the ECU uses a different data map to follow. That'* when fuel economy suffers.

So if I drive for with the downstream sensor not working, there would be no effect like low performance or high fuel consumption? Hence, it would not be the most urgent item to replace.

T.

Correct. But if its constantly throwing a code, it may throw the system into a hizzy and may not monitor somethings. ECU'* are weird like that. Tis best to just replace it. Especially if you have to get it inspected.

Quite, thanks.
I guess one cannot do much wrong with the ACDelco AFS123. It is 40$ at RA.
T.

Your welcome

Finally, I can give a feedback on this. Replaced both O2 sensors, upstream and downstream. Also reset the fuel trim history (actually, workshop has done it).

At first the motor ran really really bad, but a after half hour or so it leveled out. Now after about 10 hours or so operating, the motor runs really smooth, the slightly rough idle is gone. Also, power returned.

This is the picture of the downstream sensor we pulled out. Looks like it didn't measure anything, but it didn't throw a code either.

What symptoms were you having that prompted replacement? Just the catalyst code?