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Old 06-21-2007, 12:04 PM   #1
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Second 02 sensor

I'm about to do a tune up on my 96 Cavalier, 2.2L; replacing the coil, wires, plugs and pre-cat 02 sensor. And I wonder if I should replace the after-cat 02 sensor also. Does it affect the ECU in closed loop, or is it just monitoring the cat?
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Old 06-21-2007, 12:58 PM   #2
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Unless Chevy did something wierd, on OBD2 cars (>96) the after cat O2 sensor is monitoring the cat.
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Old 06-21-2007, 07:01 PM   #3
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if you disconnect your battery (10-15mins), i think the gm car will relearn the drive cycle with the new upstream o2 and the old downstream o2. fords have an adaptive strategy that detects new sensors and it compensates. Nissan's , not sure but usually both get replaced on the same stream so you have 2 new sensors .
the front o2 senses rich/lean as it comes out of the engine. the rear o2 senses rich/lean after the cat- at a much slower rate. then the ecu/ecm/pcm changes the mixture based on that.
i don't think you need to replace the coil unless it has a problem.
the ecm doesn't look at o2 sensors until it warms up.
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Old 06-22-2007, 08:55 AM   #4
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I replaced the precat o2 in my sunfire and the mileage returned to normal. I wouldn't worry about the aftercat. Whats wrong with your current coils? I would also test the stock plug wires to see if there bad. I ran the original plugs and wires in my wifes sunfire until the crash it had 180k. The oem wires are made by packard, I rarely find a bad one.
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Old 06-22-2007, 08:56 AM   #5
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Where are you in KCMO at
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Old 07-02-2007, 01:30 PM   #6
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Sorry for the delay in responding folks. Thanks for the replies.

There's nothing wrong with the coil. It's just 11 years old, so I thought I would replace the whole ignition system as I did the tune up.

ffvben - thanks for the tip. I'll disconnect the battery for a bit. couldn't hurt ;-)

lovemysan - I'm in North KC. working in Lee's Summit. I work for that big telecom in town, but I would quit if they wanted me to commute the campus in Overland Park. ugh.
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Old 07-07-2007, 11:02 AM   #7
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ffvben , I disconected the battery for an hour or so. Apparently the ECU did have to relearn to average the signal from the new O2 sensor; the GPH and MPH bounced around for two days while I held a steady throttle position, then settled down on the third day. The scanguage is reporting better MPG this tank, though I don't know if it is from the new O2 sensor or my getting used to using the scanguage for instant feedback on my driving style. Either way, thanks again for the tip.
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Old 07-07-2007, 06:33 PM   #8
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I believe the second O2 sees how the cat is working. If it's not burning hot enough, it tells the computer to throw more fuel in to the fire. Not sure if it's better with or without it.
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