see your new ..welcome...sorry to hear about dog,,we are pet owners ,,,
you need to get your codes read,,regardless of what you have been told by muffler shop,,, forget them,,,stay here and you will learn,,,
start with that first,,,
take some time and read about car in stickys above threads,,,
read the ford service manual to understand...codes have pinpoint test to follow for repair...
heres 2 links...
Gale Cengage Product Failure
Ford Service Manuals - 2005