355 ECU's

They're mounted on the inside of each rear wing.

On the left hand side if you look down near the area of the rear lights you will see the silver braided thermocouple wire that connects the cats to the ECU.

Never looked at the right hand one but if you follow the thermocouple on that side you can't go wrong

The ECUs are small black boxes about 3 inches high.

It's not guaranteed to be the ECU though as the thermocouples, amongst other things are known to fail.

 

Check with your dealer if it is the ecu it may be covered under the emission control warranty. But most likely the slow down message is generated because your cat(s) are getting over heated.