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Old 03-18-2012, 02:27 PM   #1
cheapybob
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2000 SC1
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Default Do I need to reset the PCM and relearn if I replace the front O2 sensor?

I'm hoping I don't need to. It seems to me that anytime I reset it on my Saturn's in the past, it takes weeks for the gas mileage to recover.
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Old 03-18-2012, 02:30 PM   #2
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Default Re: Do I need to reset the PCM and relearn if I replace the front O2 sensor?

No, just change the sensor as there is nothing permanently learned relating to the sensor.
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Old 03-18-2012, 02:39 PM   #3
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Default Re: Do I need to reset the PCM and relearn if I replace the front O2 sensor?

Doesn't matter. If you recall, open loop mode doesn't have the O2 sensor as an input until the sensor reaches operating temps where its signals are valid. Its at that point when valid O2 sensor signals are used that the pcm exits open loop and enters closed loop mode. The O2 sensor is the last one to the dinner table while everyone waits (open loop). The short and long term fuel trims will adjust automatically as will the pcm when it sees new O2 signals. It would seem that resetting the pcm manually will hasten relearning against leaving things alone and allow it to adjust automatically. My personal guess is to leave it alone for a quicker fuel trim adjustment in a shorter time span. Your choice.
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Old 03-18-2012, 11:27 PM   #4
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Default Re: Do I need to reset the PCM and relearn if I replace the front O2 sensor?

Thanks for the replies. I didn't reset it. I made the mistake AGAIN of changing more than one thing at a time (changed plugs and O2 sensor), and the engine started missing under load.

I hooked up my A/F gauge and it confirmed that the O2 sensor was at least cycling properly, and I removed the plugs and they looked fine, but on two wires, when I tried to pull it off the plug the crimp connector under the boot was left on the plug after I pulled off the boot and wire.

It turned out that the plug wires were crappy and the connection to the wire under the boots were coming loose, I guess not meant to be taken apart more than once.

Anyway, new plug wires fixed the miss.
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Old 03-18-2012, 11:39 PM   #5
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Default Re: Do I need to reset the PCM and relearn if I replace the front O2 sensor?

Cheap wire sets are not low cost but they are cheaply made and fail rapidly. You are right they may be safely connected once and after that all bets are off.
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Old 03-19-2012, 04:26 AM   #6
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Default Re: Do I need to reset the PCM and relearn if I replace the front O2 sensor?

Yeah, you buy a used car with 95,000 miles on it and you just don't know what's been done to it, or if they bought decent parts to put into it or not.

I could have tried to restring and recrimp them, but junk is junk... I was lucky they literally fell apart to clue me where the miss was from.

I also had to return a brass tipped ECTS today that always read near 55 degrees, too. It was from Autozone branded valuecraft truguard or something like that.
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Old 03-19-2012, 10:33 AM   #7
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Default Re: Do I need to reset the PCM and relearn if I replace the front O2 sensor?

If you examine the usual method of making the terminal end connection to the wire you will find that the carbon impregnated fiber is folded over to lay on the outside of the cable, a "U" shaped wire clip is jammed into the center of the cable and also lays along the outside. Then the actual brass plated steel or low grade brass terminal is slipped over the end of the cable and crimped. This makes a poor connection at best as the carbon powder eventually falls out of the fibers and you are left with an open circuit to the actual spark pulse.
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Old 03-19-2012, 11:16 AM   #8
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Default Re: Do I need to reset the PCM and relearn if I replace the front O2 sensor?

These were so cheap the terminals weren't even brass plated. And yes, otherwise they were as you described.
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Old 03-19-2012, 11:32 AM   #9
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Default Re: Do I need to reset the PCM and relearn if I replace the front O2 sensor?

Well, yes, plain bare recycled melt steel is also use on the bottom of the barrel cables. You pay good money for stainless steel with spring connectors and molded on boots. The mid priced cable set is not those but they will usually last as long as the plugs if you are not continually disconnecting them. The local FLAPS dose not stock top end cables. They do sell the fuzzy dice equivalent at inflated prices though.
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