94 Supreme codes?

So I have a 94 Supreme with a 3.1
Ever since about 105000 mi (124k now) the service engine soon light has come on intermittently. At first it was only at highway speeds and engine warmed up and would come on if I took my foot off the gas (coast to intersection.)
Lately it comes on quicker and more often.
So i got the Chilton's manual from the library and it said there is no way to pull codes???
The connector under the dash looks like an obdc1 type with two rows of six slots, but only three of the slots have terminals installed.

So is there any way to pull a code at home?
AutoZone has already confirmed they cannot read codes on this year/model.

Car does run fine except somewhat high idle. Got a tuneup a couple months ago. Was hoping the light would stop lighting up.

You need it scanned withe professional scanner that will work with that transition year called obd 1.5.

So should I assume this needs to be the dealer?

I did get codes scanned by mechanic a few months ago. He said EGR valve had a code and that it would also explain the high idle I was having.
I did the replacement of the EGR. The idle immediate improved.
The check engine does still come on occasionally but is back to only when driving on highway and then lift foot off of the gas completely. I let it go for a couple months just in case the computer needed to relearn before resetting itself. I typically don't do much highway driving so that also factored into my not reporting back sooner.

I had read somewhere about cleaning carbon deposits from the passages. I did not see anything on the two passages on the sides under the EGR. There was one port in the middle, but if I remember correctly it was blocked off from the bottom. I am guessing it is some sort of spring loaded cap from the bottom. I did not try to force that open to clean it. Should I do so? If so please tell me what needs to be done as I have not done that type of thing before. If not that then any new suggestions are appreciated.

When you take your foot off the pedal, the ECM does a test.....it opens the EGR very slightly and looks for a change in the MAP sensor....if it doesn't see it, then the code sets......what usually causes this, is the flashing in the passageway of the upper plenum has a build up of gunk on this flashing, and causes a restriction, such that the MAP can't sense the pressure change with the slight opening of the EGR.......that passageway has to be cleaned out......have seen some so bad, that the upper plenum has to be removed and given a caustic bath, rinsed and blown out.....

Tech ll,

thanks for the response and the info of what the ECM is doing and looking for.
is this something that can be done without removing the plenum? Is it OK to the to push the "valve" for the exhaust passage to try to get it from the outside?