Oxygen sensor operation

[obadran]

I have a general question on how O2 sensors work.

96 Yukon 110000 miles

Is it normal for both upstream sensors to output zero volts under a hard accelaration (ie. trying to merge into 70mph traffic)?

I'm having this problem with a lack of power and I noticed that when I "punch it", the engine hits somewhere between 3000 and 4000 rpms and then starts hesitating and VERY SLOWLY the rpms climb...also during this little test, I notice both upstream O2 sensors drop to zero volts and the computer goes into OPEN loop mode. If I do this test long enough, the SEL comes on with P0131 and P0151 codes (O2 sensor low volts sensor 1 banks 1&2).

Could this be faultly O2 sensors or could it be a fuel delivery issue that runs the engine so lean that the sensors are reading correctly?

I'm also starting to wonder if some of the sensors are starting to fail given they are all 110000 miles old.

Thanks,
Omar

[94 Jimmy]

Quick reply, if you haven't replaced the O2 sensors in 100KMi do it.
The oxygen sensors are actually batteries which generate a voltage in relation to the amount of oxygen left in the exhaust after combustion. Generally they should read between .4 and .6 volts, in fact if they don't transition between these two points often enough in a specific period of time the computer will set a code.
As they age, the sensors can drift. They still seem to be working, but give an overly rich or lean reading. My sensor tended toward the rich, the computer corrected the mixture toward lean and under certain conditions the engine would lean out to the point where it just quit. After a period of time the computer noted that there were too few transitions from rich to lean and set a code. A new O2 sensor fixed the problem.
The upstream sensor (in front of the CAT) actually controlls the mixture, the downstream sensor tells the computer if the CAT is working. If you can't afford to change all 4, change the upstream first and see if that does it.
Be careful when measuring the voltage output on these sensors. If you use a low impedence meter you will draw two much current out of them and destroy or damage them, changing their output. Better to get an OBDII decoder that lets you read the codes and sensor data on a computer. Check out Scantool.net for a good low cost decoder which not only reads and clears codes but allows you to display all of the sensor data on a computer, laptop or PDA.
Hope this helps.
See ya
94