91 S-10 ses light comes on.
 

i picked up a 91 S-10 4.3 automatic with 72,000 miles on it for my neighbor last spring for $700.

did a tuneup on it, and threw a muffler on it.
he just called and said the SES is coming on occasionally and the fuel mileage has gone down.

a few questions.
where is the diagnostic port on this truck, and will i read a code without the light lit?


thanks, Tom

Welcome to the forum...
The OBD port should be under the drivers side dash...
You will need to use a paper clip to short out terminals A and B with the engine off key on...
The SES lamp will eliminate and flash a series of blinks, a blink code of 1-2 will happen 3 times, after that 3 times any stored codes will "blink out" so you will need to be diligent about counting the blinks....
Once you are sure you have them all and are certain of the codes, you can clear them by disconnecting the battery for several minutes....
Sometimes its best to read them, clear them and see what comes back as some of the codes can be old or stale and not present anymore, meaning the problem is gone but the code can still be there...
Here is a good source for reading the codes How to read older GM OBD diagnostic codes for free
And here is a list of the codes for the OBD1
GM Trouble Codes - 2 digit codes

Please do follow up on what you find, we'd like to help you get it fixed !

will my scanner with the gm OBD1 adapter read the codes?

I have never used a scanner for OBD1 so I can't answer that one, but if you have the adapter I'd give it a try for sure...

well, i finally got hold of the truck this afternoon.
came up with
code 44 : O2 sensor lean
code 45 : O2 sensor rich

he said the light comes on around 2 miles out when cold started, and again about 6 miles later.
both times the light stays on around 30 seconds or so.

truck got plugs wires and cap around 2000 miles ago, and new air filter on monday.
i also put a cat back exhaust system on it thanksgiving weekend, and the CEL started 2 weeks ago. .