Fuel Injection Control Unit Diagnostics

["Jose A. Ceroni" <ceroni@ecn.purdue.edu>]

I have been tracing down a high idle in my 88 90Q. From pulling out the
codes, I got 4431 (ISV) and 2123 (Full throttle switch). However a separate
test of the ISV reveals that it works. The switch on the other hand, seems
to be stuck in the open position (I checked the switch and it works). All I 
have left to check is a possible faulty FI control unit. Has somebody 
performed a test of this unit before? Any recommendations and an estimate 
of how big the hole in my wallet will be if I have to replace it? TIA,

J.A.Ceroni