open & close loop

1996 S-10 2.2,I recently had a 141 code O2 sensor at the rear of the catalytic and after replacing the sensor, I notice the scan tool was then showing close loop, but moments later " 5 mins" it would go to
open....... monotering the LT trim was at 114 and the ST Trim it was 128.... the O2 sensor at the rear of cat was in the .833 mv all of the time. Should the voltage toggle for the sensor ? keeping it in close loop ? The code has not returned since replacement of sensor, engine temp is at 207 F .. seems to operate fine.
Thanks, Marc
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When cold the engine runs in closed loop using preset defaults for fuel trim. As the engine warms up it goes into open loop where the front O2 sensor and other conditions control the fuel. The O2 sensor after the converter is used to determine the efficiency of the catalytic converter and does not switch much. If your OBDII scanner has the ability graph the two O2 sensors while driving and you will see the relationship. If the aft O2 sensor starts switching as much as the front one you will get a cat efficiency code....

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Woody, you got it backwards. Closed loop is under the control of the oxygen sensor.
Woody wrote:

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Update for more info, 130,000 miles but in superb condition, start up is good "open loop", 3 to 5 mins later, light comes on and PID shows "close loop"........then in about 5 mins time, returns to open on PID and light on the snap-on scanner. engine temp 207............ monitoring the proccess from cold to hot front O2 around 500 mv when hot, switches more when cold rear O2 steady at .837 when hot............ starts cold with lower mv like 190, 288 LT @ 114 ST @ 128 at 50 mph injector pw 3.9 idle @ 1.9 all inj. equal on all the injectors no DTC's since the O2 has been replaced. if the Cat wasn't operating properly, could that cause the rear O2 not to switch, and being in the open loop, is that why the LT Trim is at 114 ? But here is what is confusing to me, when the engine is cold "open loop" I see no PID for the LT & ST trim, which is okay. After it goes to close loop, I can then see the PID for the LT & ST trim...... but when it returns to open loop the PID for the LT & ST continue there display. So it's obvious that it is still in close loop if the LT & ST are still displaying, no ? Thanks again. Marc
snipped-for-privacy@idirect.ca wrote:

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In article
snipped-for-privacy@transtechca.com wrote:

No, not a post cat O2 sensor. A wash coat of cerium is added to OBD2 catalytic convertors, the cerium stores oxygen, a high voltage reading from an O2 sensor equals low oxygen content in the exhaust, the steady high voltage reading is telling you that the catalytic convertor is still capable of storing oxygen. periodically, you may see the O2 voltage go low, this is the cat burping off excess oxygen, completely normal.

Rear O2 has no closed loop authority.

The P0141 was for a heater circuit failure. Good odds that the rear O2 sensor internal heater burned open and replacement fixed the problem.
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Thanks to the tips everyone posted, the FIX was the front O2 sensor, when it went back to open loop, I noticed the mv in the front O2 was fixed at .535mv and did not switch. I purchased a new one, installed it and now the close loop is steady without dropping out. Thanks again, Marc aarcuda69062 wrote:

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