From: mshar@mentor.cc.purdue.edu (Mike Mshar)
Subject: Re: Service Engine soon light coming on
Organization: Purdue University Computing Center
Distribution: na
Date: Fri, 6 Nov 1992 22:10:51 GMT

> ec@fc.hp.com (Edgar_Circenis) writes:
>>Doug Geiste (geiste@hsi.com) wrote:
>> I'm hoping that someone can point my the right direction for this problem.
>> 
>> My 1987 Pontiac Grand Am (4 cyl) has a problem with the Service Engine
>> Soon light comes on. The car runs fine though except for one noticable thing.
>> 
>> I was sitting in the car with the engine running waiting for the light to
>> come on (my wife said it takes a few minutes after the car has warmed
>> up before the light comes on). Just before the light came on, the engine's
>> RPM went up slightly. I could almost smell something like sulfur. Sounds
>> like catalytic (sp?) converter. Could someone tell me what the possible
>> causes are of this light coming on? The air filter is new so it's getting
>> air ok. It's been a while since it's had a tune-up though. The light 
>> will go off/on intermittently while driving the car.
>
>Well,  I have an '89 Gramd Am and experienced the same problem with
>the "service engine soon" light.  I ended up taking it in for the 30K
>checkup and asked them about it.  I cannot rembmer what the problem
>was, but it was a problem.  This is not one of those "remember to get
>your car serviced" lights.  If it's on, there's probably something wrong.
>
>(It wasn't the catalytic converter in my case)
>
>Edgar


Well, what that means is that the computer has detected something wrong
with itself. It DOES NOT necessarily mean that the engine itself needs
work, but more than likely something computer related does. 

No, it is not a light that tells you that it is time to change the oil.

The light will come on whenever a trouble code is stored in memory. This
trouble code is stored whenever the self-diagnostic program in the computer
finds something wrong with one of the parts of the computer system.

Some examples are...

Bad Coolant Temperature Sensor
Bad Oxygen Sensor
Bad MAP or MAF sensor (Manifold Absolute Pressure or Mass Air Flow)
Bad PROM chip

etc, etc.

What should you do??

If you are good with tools and such, buy a Haynes manual for your car. But,
before you buy it, make sure it has the chart that lists all of the trouble
codes for your car. If it doesn`t, (and some of them do not have this chart
for various reasons), go to the library and find a Haynes or Chiltons guide
that does. I know for certain that the Haynes Guide that covers Chevy and
GMC full size trucks has this chart, because I own that manual. It does not
matter which car, because my mechanic told me that all GM computer trouble
codes are the same for all GM cars. In this section, the procedure needed
to read the computer`s memory is outlined. The computer will tell you what
trouble code it has stored, and all you then need to do is simply find the
code in the chart. For this part of the procedure, no special tools are
needed, just a short length of wire to ground a certain terminal.

Once you find out what the trouble code is, you will know what is wrong
with the computer system. (Editors note: this is an excellent way to test
your mechanic. If he tells you the engine needs lots of mechanical repairs,
he is full of it). Then you can go from there.

Or, if you don`t like the idea of probing a computer, take your car to a
mechanic WHO HAS THE EQUIPMENT REQUIRED TO TEST GM COMPUTERS. Can`t go
wrong if you go to a GM dealer. What he will do is hook up a computer to
your car`s computer, and be able to read EXACTLY what all of the sensors
are telling the computer. From that, he can tell which sensor is screwed
up.

In my cases, I had a coolant temperature sensor go bad on my mother`s car.
She took it to a mechanic, I told him what was wrong, he confirmed it, and
charged about 100 dollars. (If I remember right, the part was 75 dollars)
The coolant sensor also went out on my TA, and again I told my mechanic that
before he replaced it. I would have done it myself, but at the time I had
no time. My fathers car had a MAF sensor go out, and the computer told me
that, too. So, it can be done, and if you know what is wrong, the mechanic
cannot screw you over. 

So, to sum up, there is definitely something wrong with the car, but it is
probably in the cars computer system, and not something mechanical. It
needs to be fixed.

Hope this helps...

-- 
|       Michael G. Mshar        | My Life-Long Goal: To keep common sense    |
|   mmshar@mn.ecn.purdue.edu    | alive and keep the idiots from taking over.|
|  mshar@mentor.cc.purdue.edu   |     \\President, Nice Guys Anonymous//     |
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