Here are some OBD1 codes relative to your post:
26 quad-driver module (computer) circuit #1 fault
12 System normal
13 oxygen sensor circuit open
44 oxygen sensor lean exhaust
45 oxygen sensor rich exhaust
63 oxygen sensor right side circuit open or map sensor out of range
64 oxygen sensor right side lean exhaust indicated
65 oxygen sensor right side rich exhaust indicated
Code 26 is related to a failure in the ECM, which is related to a device (such as a sensor) the ECM controls on circuit #1. You don't want to replace the ECM until you find out what caused the failure.
We will need someone with a 91 FSM to tell us what devices the ECM controls on circuit #1.
What operating problems are you having with your car?
Please add your year and trim of your vehicle to your signature line. That way when you start asking questions in other areas of the forum, we all have a better idea on how to assist you. Click on "Quick Links" above, go down to "User Control Panel" and then click on "Edit your Signature" It'* pretty straight forward from there.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)