Oxygen sensor bad if SES light isn't on? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 12-05-2007, 09:51 AM   #1
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Default Oxygen sensor bad if SES light isn't on?

Hey all,
I know I have posted this before, but I want to make sure that I am not wasting money on an 02 sensor if I don't need one. I have a freshly rebuilt L36 with roughly 1000 miles on it, and I feel that its gas mileage has room for improvement. When I first got the car with the fresh engine in it, the mileage was really bad, but it has improved slightly. It is believed that the 02 sensor has 110,000 on it, but the SES light isn't on. Should this mean that it is working correctly, and that if I replace it I will just be wasting money? If an 02 sensor will help gas mileage, I will purchase one and install it. Other than this mileage issue, the car runs great. With gas prices being so high, I want to get this thing running really efficiently. Thanks.
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Old 12-05-2007, 09:55 AM   #2
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What kind of mileage is it getting?
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Old 12-05-2007, 10:02 AM   #3
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I would say probably around like 18 mpg with 40% city and 60% highway. Also, I am a grandma when it comes to driving this thing. On my way to work today I think the tach might have hit 2200 rpms for a split second. Needless to say, I feel that there is room for improvement on this subject. I hear guys on here saying they are getting like 18 city and 30 highway.
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Old 12-05-2007, 11:33 AM   #4
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The O2 sensor has to be dead, or on its last gasp before it will set a code. With over 100k on it, you can be pretty certain the O2 is tired, and due for replacement.
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Old 12-05-2007, 11:44 AM   #5
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A lazy O2 will not throw an SES, but will throw an error code when scanned.
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Old 12-05-2007, 11:57 AM   #6
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So replacing it will give me better gas mileage along with even better drivability? I heard to go to Rock Auto, I saw the discount code sticky up top, and was wondering which one do I buy from the list when you type in 1997 Oldsmobile 88 non supercharged? Theres several, I know mine has three wires on it and it is a heated unit.
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Old 12-05-2007, 12:53 PM   #7
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Doesn't hurt to replace it as part of a tuneup if your mileage is poor.

It was tripping a code on my 00 but I replaced it anywhere and the improvement was huge.
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Old 12-05-2007, 01:09 PM   #8
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I recommend that you buy a Delco oxygen sensor. You may pay $10 more for it, but you will have the best chance for optimizing fuel economy with the OEM. Also, be very careful with it. Keep the anti-seize lube where it belongs and don't strike it, drop it, or overtorque it. Dropping the sensor on the ground can ruin it.
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Old 12-05-2007, 01:35 PM   #9
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allmachtige did you say you did replace it due to the SES light, or you replaced without the SES light? Your sentence is confusing thats only why I ask. Thanks.
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Old 12-05-2007, 03:50 PM   #10
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Here is the deal... If the PCM sees the sensor functioning, and we are talking the totally dead side of functional( O2 On Life Support ) you will not get the SES... It literally had to be 98% history before you'll know about it..

In most cases when everything is up to Par, but the Mileage numbers still suck, then the O2 will be the way to go...


From what you are saying it sounds like you have gained a couple of MPG'* and most of that is probably due to the engine breaking in and freeing up a bit...

I have been doing a lot of Mixed driving lately, with about 30-40% City and 60-70% Highway.. I pulled a 26mpg at the last fill up.. That was on the Ethanol Blended stuff..

I'd use Rock Auto for the senso with the BC Discount code...

Either the AC Delco or Denso sensor will work for you..

Again, I can't promise that the sensor is the issue, but 9 out of 10 its usually the cause of bad mileage for the 3800'*... Both of my 3800 easily average 20 Mpg all city

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