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Service Engine Soon Warning


Well-known member
May 18, 2003
Orange County, CA
1991 Turquoise Metallic Coupe
The Service Engine Soon warning light has begun to go on in my 1991 L98.

According to the owner's manual, I should take the car to get serviced immediately. However, it doesn't explain why it goes on or what it normally goes on for.

At first, the light went on for a few seconds while driving. Later, it started to come everytime I drive the car. Yesterday, it went on and stayed on for while then went off. Then it did it again. All this while I was out driving the car long distance.

On all instances, the car NEVER ran funny or abnormal anytime the warning light went on.

Does anybody know what this light normally refers to? I mean, Service WHAT soon? What part of the engine? I'm afraid to take it to the dealership and have the mechanic scratching his head thinking the same thing I am: "Service what?"

The car has 43,000 miles, an L98, automatic.

Any help would be greately appreciated.
Service Engine Soon Light

Hey Rogue, hello from suburban Chicago. I think I can shed some light, no pun intended on your service engine light issue. My first Vette was a 91 ragtop and I had this problem as well but it was with a few more miles on the clock, around 60,000. This warning light usually involves some part of the emissions system. It could be an oxygen sensor going bad, you have two of them, I believe or it could be a bad EGR valve. If the sensor or EGR are becoming clogged, the computer is throwing a code. It might not effect performance for a while but it will eventually. I don't think you have a major problem but it is one you should get resolved. I hope this helps. Neither part is really expensive to purchase and replace. I also had the same problem with my 96 Coupe and it was the EGR valve.


You can retrieve these codes yourself. Your car should be equipped with the OBD I system (on board diagnostic) You can go to autozone and get a key, or just use a piece of wire to short two terminals of the diagnostic connector under your dash. When sitting in your car, the terminal is above your right knee. Let me look in my service manual, I think it is terminal A and ??. I cant remember but I will look. It is very simple. Short the two terminals, turn the ignition key to the on position, do not start. The cooling fans will come on. The service engine soon light should be on. It will flash the code 12. Which is on flash, pause, two flashes, pause, one flash pause, two flashes. This will occure three times. After the third time it will flash the trouble code. It will be a number. Then you can relay the number of code to the specific problem.

Okay the terminal will have slots.

here are the positions

G and these it says is open.

This is for my 86, but should be the same. There is a little A over the A slot. Short with a jumper wire slots A and B which is really just putting B at a ground potential, becase slot A is ground. This will force Diagnostic Mode. From that you can retrieve the codes like I explained in the previous post. If you dont have the codes, you will probably be able to find them on the net, or post your code and me or somebody else will be able to tell you what it is.

Good luck

Rogue, I had a similar problem with my car and got it fixed by replacing the O2 sensors. We used a scanner to identify the problem rather than using the diagnostic terminal inside the car. I think my error code was 44 or something like that.

The nice thing about the "higher end" scanners is that you can (or rather your mechanic) can perform a thorough check of the engine and confirm all parts / systems are working okay. You can even ask for a compression check, cylinder by cylinder... It is a 5 minute job. That's what I would do if I were in your shoes.

Hello ! I am new guy here , haven't introdused myself properly yet , but that's due to no pictures of the car yet . But I will make'em soon .
But I have a similar problem . When driving out of the town for prolonged period of time , same light goes on . previous owner said it's due to some disconnected plug of emission control system ...
Basically , I was told , I could get rid of all those pipings , and get some torque with it , but I think soon we'll have to face strictlier control of emission systems , so I'd better leave them on for a while .
But , the question is , shall I connect it or shall I leave it like it is . Maybe my sensors are screwed and that's why it was disconnected , to make vette drive more or less ... Or ? Maybe you can share your opinion on this subject , cause I don't understand everything yet :)
Car is 91 L98 just like SSTibet's :)
Dear Max,

Believe me you will never understand everything regardless of how long you own this car. But let me tell you this; what makes the engine compartment so complicated is the emission control systems... Assuming that you are a new owner, I highly recommend to take the car to a specialist with the scan tool. Let them scan the Service Engine Soon (SES) warning and also give the engine a complete check. I do not know if you had this experience before, but they can perform a complete check of the egnine with high end scanners. It is like a check-up in a hospital (fortunately not as complicated and expensive). And it does not take more than 5 to 10 minutes for all of that.

Give it a fresh and good start is all I am saying. It is next to impossible to guess warning lights in absence of the scan tools. An exception is the ALDL connector (diganostic link below right knee bolster) which will tell you the error code but will not do the other checks.

And I would keep my emission system components intact; would not remove them. All you will save by removing them is probably 10 - 15Hp. Will you really consume all the Hp you have in that car & need the last few bit of power???

Cheers & welcome to CAC
Thanks for quick reply . But , what is this scan tool ? A computer with special plug and special diagnostic program ? I have seen something like that in a local mercedes joint , my friend works in . they run some tests and check for irregularities . That thing alters the rpm and all kindsa freaky stuff ...
but anyway , I am always trying to understand how things work , and how you can repair them in case of need :) I 'll leave those pipes on , but still don't know 'bout that plug ... what plug is that anyway ? Guess have to dig in and check where is it going to .
To Rogue and Max --- A really good thing to have is the factory service manual for your car. You can order one of these from www.helminc.com and it costs about $100. Even if you don't intend to work on your car yourself, it is a good thing to have around. There is lots of info about your car that is not mentioned in the Owner's Manual (by the way, you can buy one of these from Helm's also!).

My fsm ('87) gives a nice overview of the types of information available from the diagnostic connector and then gives detailed instructions for testing all of the problem codes that the onboard computer can store.
The "plug"? I assume you mean the diagnostics link connector... To find it is not as easy as it sounds for a new user. Position yourself in the driver seat, take a mirror and hold it slightly below your right knee level, illumunate the right knee area with a torch. You will see the connector. Once you find where it is you will then have to figure where A & B positions are on the connector. Then follow the steps described in other posts in this thread.

Better for someone like you is to purchase the Chilton & Haynes repair manuals for C4 Corvettes. You can get them from www.ecklers.com or maybe from a local bookstore. Since I live in Turkey I cannot tell you which is faster.

And for the scanner; you can read about them in the manuals I have mentioned above. But there are the "basic - simple" ones that tell you what the error is. And then there are the "more sophisticated" ones that tells not only the error but also takes measurements from all sensors / measurement points. It can also run a number of tests... :)

When I had my car scanned they had the "more sophisticated" one and the mechanic told me he ran a cylinder compression test, one cylinder at a time... So you can see the error codes, readings from different parts of the car and also run tests with this sophisticated device...

Oh , I am sorry , I guess I made myself not clear .. I meant the connector which is somewhere somehow connects something in that emissions control system .
I understood the way to find the diagnostic plug under the dash .
It's just the previous owner told me that some connector is disconnected and that's why when driving 60mph for some 1 hour the SES light turns on.
I guess I'll better ask him once again what is that connector and where it is ... I just hoped somebody already knows :) lazy me :)
Yeps, you got to do that buddy. Ask the previous owner what has done. A lot goes into the emissions control systems of your car.

My friendly advice? Put the car back into its original condition and do not tamper with anything unless you know exactly what you are doing. If you of the "try and see" type of person then you need a lot of nerves to fiddle with this car...

Cheers :)
Make sure the gas cap is tight....it could be something as simple as that. My light used to sporadically go on and now I give the cap at least 3 clicks and I haven't seen the SES light since.
Great responses! Thank you all.

I thought it may be emissions related. The Owner's Manual HINTED at it but didn't say that was it. Go figure. At least now, I know where to get started to get this thing rectified. Might as well change the oil and filter too.

Thanks again everyone. I'll post a response this week when I find out the results. :upthumbs

It is probably the EGR that is causing your problems. If it only occurs on the highway, the EGR switch might be what the previous owner is talking about. I dont think it will do any harm, your car probably just produces more emissions. I would get it fixed though.


Have you ever gone into a shop when they were working on your car. Most likely they had the factory service manual out, and were referencing that while working on your car. I know my shop uses them, and they specialize in vettes. Dont you have OBD II on your car. It is slightly different then the OBD I setup. Yes you can put the scan tool on a car, but at what price.

Do not purchase the haynes or chiltons manual for your car. IT IS A PIECE OF JUNK. It covers the years from 1984 to 1996. Big time frame. The books are a total waste of time. I bought my service manual for 42 dollars and it is in great shape. You can buy it from helminc, but look at ebay too. I think even a beginner should be able to retrieve codes stored in the computer.


I was with the technician here when they scanned the engine. We found the O2 sensors were not working and replaced them. In most shops here the customer pays a "reasonable price" for labor... Most sop owners will not tell you how much, believe it or not. I think I paid them something like US$30 or so for O2 replacement labor and the scan. :)

And you are right for the Haynes & Chilton books. They are good enough for an enthusiast like me. But if I were to perform work on the car I would buy something more reliable. What manual do you have exactly?


You got off pretty good with the labor on your car. I bought the factory sevice manual for the 1986. It has 3000 pages specific to my car. Wiring diagrams, troubleshooting, and all other kinds of stuff. If you can get one resonable for your car it is a great investment.

Craig, thanks much. Factory service manuals are expensive. I checked the helminc for 95 corvette and price is like $130. By the time it gets to Turkey by airmail it will become more like $180 - $200. Not worth it when I drive this car like 1,000 miles a year...

Still, is there anyway I can get this cheaper???
SSTibet....IMHO for your needs a Chiltons or Haynes for 15.00 on ebay will do just fine. It may not be year specific for just 95's but it covers the LT1 and the later model C4's adequately and is a helluvalot cheaper. Who knows...you may run across a 95 service manual eventually, at a deal you can't refuse, but in the meantime your covered.:Steer See ya
PS Great pic of yor car....that yellow looks good!

Did you check ebay? If you aren't really interested in learning about your car the Haynes manual is fine. If you want to know your car, and be confident about repairs the service manual is the way to go.


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