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Chad:

Here is a car question for you.

I have a 97 Chevy Venture, 3.4 with 72K on it. While driving up to Canada the check engine light came on after 7.5 hrs of driving. It ran ok for a little while but coming into town it started to buck and stall out on idle. You could smell gas from the exhaust (running rich.) In town there is a local mech. that hooked it up to the scanner and found a fault with the S1 O2 sensor. My son, who is also a mech. by trade, was looking over the shoulder. The tech ordered the part and I went back a few days later to have it replaced. On the way to his shop the car ran fine. The sensor he replaced was the O2 sensor behind the converter which my son said was not the S1 sensor. He cleared the computer and we went on our way.

Driving home, almost like clockwork, at 7.5 hrs, it started to do the same thing again. I was going about 75 MPH most of the way with the a/c on so she was under a load. 92 on the gas, no cheap stuff.

Started the car this morning, went to work, did some freeway driving and everything was fine.

WTF?

Thanks in advance.

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Hi Terry,

The s1 sensor is the primary sensor. The mechanic replaced the post cat sensor.

The primary sensor function is determining how much oxygen is in the exhaust, thereby determining how much was use in combustion. The information that the sensor sends back to the computer alters future fuel trim. "Future" means the next one or two tenths of a second.

The ideal mixture can't be obtained because of constantly changing variables like temperature, load, throttle position, etc. Instead the objective is to constantly trim fuel mixtures to the anticipated demand. Fuel trim on your car changes ten or twelve times a second. It's always rich or lean..but not for long. By switching rich or lean at a fast rate, a close facsimile to stoichiometry can be achieved.

The sensor the mechanic changed is there to make sure the catalytic converter is doing its job. If the catalyst is performing properly there should always be very little oxygen in the after cat exhaust because it all got used in the cat to complete the burn.

Primary sensor readings oscillate below and above .5 volts 8 to 20 times a second. Post cat readings theoretically should always be below

.5 volts.

The primary sensor is active in controlling vehicle performance, the post cat sensors are baby sitters that just monitor the emissions systems.

There's a little more to it than what I've described, so if you want give me a call @ 585.865.7553 and I'll be happy to be the one that knows something for a change.

At any rate, if the code said S1..the guy replaced the wrong one.

I hate to say this because it'll confuse things further...just because the O2 code appeared, doesn't mean it's the O2's fault. The code means it's not switching above and below .5 volts often enough. The sensor could be causing the problem or the sensor could be doing its job and reporting what it sees and the code may be symptomatic of another problem.

The fuel pressure regulators go bad on that motor, leaking fuel into the vacuum line, so the darn thing will run rich all the time and the sensor will report it to the computer and the computer will still say the sensor's bad.

Replace the primary sensor first, then check other things if you still have a problem.

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Originally posted by Chad Fabry

I hate to say this because it'll confuse things further...just because the O2 code appeared, doesn't mean it's the O2's fault. The code means it's not switching above and below .5 volts often enough. The sensor could be causing the problem or the sensor could be doing its job and reporting what it sees and the code may be symptomatic of another problem.

Replace the primary sensor first, then check other things if you still have a problem.

There goes Chad again, quoting code!!![:-magnify]

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