The Caddy ECM that you are installing in your Fiero will operate like the Caddy ECM it is.
The Caddy's BCM (Body Control Module) was able to display the trouble codes on the heater control's display without shorting A-B. Because of that GM didn't build in the A-B shorting diagnostic routine into the ECM. So you lose that simple diagnostic ability. Sorry.
You can however use a Red Devil River 12 pin ALDL USB cable, TunerPro RT and a Windows laptop to read any codes and also scan all the ECM inputs and outputs.
You can also do it using the Red Devil River 12 pin ALDL USB cable, ALDLdroid, and an Android device to read the codes and also scan all the ECM inputs and outputs. Note your Android device has to support host mode.
You can also do it using the Red Devil River 12 pin ALDL Bluetooth adapter, ALDLdroid and any Bluetooth Android device to read the codes and also scan all the ECM inputs and outputs.
Red Devil River ALDL USB cable - http://reddevilriver.com/aldl.html
= Comes with free version of TunerPro RT on the disk - Shameless self promotion - This is me
Red Devil River ALDL Bluetooth adapter - http://www.reddevilriver.com/ALDL_Bluetooth.php
- also comes with the free version of TunerPro RT - again a shameless self promotion
ALDLdroid - https://play.google.com/sto...om.sgiroux.aldldroid
Also - The Caddy ECM's ALDL connector puts the data on pin M. GM did this with all the newer (1990 ish +) OBD1 ECMs. Pin M is a bidirectional data wire. If you have the simple one transistor ALDL cable that was wired for the Fiero PIN E ALDL connector, it won't work for this ECM. You also can't just move the pin to match up to make it work since the one transistor cable is only capable of unidirectional data flow and not the bidirectional data flow needed by the Caddy ECM.
Make sure you run a switched +12v wire to the Power Steering Switch input on the 4.9 ECM.
[This message has been edited by phonedawgz (edited 05-21-2014).]