strange check engine light

[WEG1192 at aol.com]

Ok, I've got a problem I haven't seen here before. I had to transfer the ESC 
and netres chips from a "bad" 27256 memcal to a good memcal. When I crank the 
engine, the SES light comes on rather quick and the engine idle is way up 
there. Sounds like the idle air control is wide open with all the hissing. 
When I install the field service mode jumper between ALDL A and B (engine 
running), the light goes out and the idle drops to normal. I remove the 
jumper and it runs fine from then on until the next crank. The code I get is 
code 51, which I believe is an EPROM code. What does putting it in field 
service mode have to do with code 51 and why does it fix it.  This is a 
strange one.  JW
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