First check the connector thats underneath the box. They are known to wiggle loose. Several people have also said you can try to resolder the connections within the box.

Has the mechanic ruled out the idle valve itself, or other electrical problems?

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma