2001 Cadillac Deville Check engine light

We have a 2001 Cadillac DeVille we bought new. Missouri has inspections including emmissions. Our car failed the inspection as it has the "check engine" light on. A local Cadillace dealer tells us we need a new computer and the transmission has to be dropped, a 12 hr job, and have a torque converter or sensor replaced. The total estimate is $3700 plus tax will go over $4,000. Is there a cheaper way to do this. I am told it has nothiing to do actually with emmissions but the State requires if this light is on to have it fixed (turned off) before it can pass the emmissions test. Can you help? We are senior citizens and on a limited income and $4,000 will hurt us greatly.