WARNING: This page is about a different car, the 1990 Cadillac Fleetwood and 1990 Cadillac DeVille. However, it is still accessible from the selected car via links, so may be relevant.
Replace fuse, start engine and allow to idle. Manipulate circuits No. 120 and 139 wiring. Look for shorts to ground. If fault is induced, the fuse will blow and Code E20 will be stored as a hard code.