My 2010 legacy GT HAS 145,000 miles on it. It’s driving great but, the gas mileage is terrible and my dash is lit up like a flashing Christmas tree. The local mechanic said Cat needs replacing but wants me to come back to see if there’s more because of the flashing lights. I have days where no lights flash but it comes back. Any suggestions? Could it be sensors? If it is the Cat where should I get a new one besides the dealer