misfire or ecu problem

Hi i have a 2000 dts with 103000 miles on it.The other day car was driving fine came to a stop light and car loss power had no power get down street.Engine sound loud like exhaust leak.Service engine light is on went to a shop said it has a misfire it needs a major tune up called shop today now they say it needs a new ecu that was the problem.What sign do you get when ecu goes bad.

Normally ses light, there is no for sure sign other than running left front of car to to solid object because its mounted under air cleaner box.LOL