I forgot to mention that was the location for 92-96. this is the location for 85-91, same side of the truck, right behind the air cleaner

Thanks for posting that, quick and clean, great photos.

good info here thank you

me thinks you got the locations crossed. all my 87-91 trucks the test port was in the back near the hinge..

Diesels are in the back like the newer gas ones Tom.
There is no room for them by the air cleaner because of the 2nd battery

gotcha!!!! makes sense, i only had the one 89 gas truck at the body shop and only opened the hood on that one time to repair the busted fuel pump relay wiring.

OK, I see how to check which code is thrown but how do I clear them (did I miss something)?

Once you repair the problem, the code will become a soft code, the light will go out. Computer will erase the code in 60 key cycles

Or
You can unhook the - battery cable for 5 minutes, but it also removes some memory.
That takes 60 key cycles to reload
Either method is acceptable

Thanks again!