98 Caravan Service Engine light - mileage or exhaust?

Hi I have a 1998 Caravan with 117960 miles. The "Service Engine Soon" light just came on yesterday, at about 10 to 20 miles less than stated
above. Before I go for the diagnostics somewhere, I read that certain vehicles are programmed to have the SES light come on at a certain mileage. Is mine one of them? The other obvious thing is I have a hole in my muffler/exhaust system somewhere, that I plan on getting fixed soon. It wasn't too bad yet the last couple of weeks, but is starting to get a bit louder. Will the computer sense that? I just want a clue, if possible, what I'm dealing to ruke out the above two things before I start spending money on repairs. Thanks much.
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I found my answer, so no need to respond. With more Googling, I found people were getting a free diagnostic at their local AutoZone stire, so I tried that, and my local store diagnosed it for free with a handheld scanner. It's an O2 sensor that's high on it's voltage.
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That's not a diagnosis, that's merely a trouble code extraction. There are a number of causes for high O2 sensor voltage.
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Thanks. BTW, I just had a new muffler put on, and the SES light is no longer on.
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I doubt that fixed it. The muffler is too far from either O2 sensor to have an effect and even if it weren't, an exhaust leak would drive the O2 voltage low, not high.
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Yep, I spoketh too soon. The SES light came back on today. Wonder why it went off after they installed the muffler, it went off after driving a mile out of the muffler shop, and stayed off two days, now back on. Guess I'll have to replace that O2 sensor, or have somebody look at it to see if something else is causing the sensor to run high. Thanks.
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