can having major service and tune up cause a p0420 code

Question

I brought my 2013 ford fusion in for service and had a bunch of stuff done at 120k miles.  I had the battery replaced, trans fluid replaced, coolant replaced, spark plugs replaced, fuel filter replaced, injectors cleaned, air filters replaced, and an oil change all at the dealer. 2 days after having that done (maybe 50 miles or so) my check engine light came on, and it did run a little funnny, so i stopped back at the dealer and had a stored code for thermostat slow to respond.  had that cleared and a few days later the light came on again.  this time for p0420.  Engine runs fine, but it has came on and been cleared 3 times now over the past 3 weeks.  

I am thinking that alll of the learned data was not erased and the ecm is going crazy after the service.  I brought it to the dealer and they said I need a new Cat.  I have never had a check engine light come on sonce I bought the car with 4 miles on it.  now after having it serviced, it wont shut off.

Answer 1

Rarely it can, since you went the dealer , they can only use dealer parts, All this started all due to the battery getting disconnected. When the battery was disconnected. The fuel trim is reset, all the monitor had to run to complete. If the code before the catalytic code was for the thermostat, is the radiator full proper level. The best way to determine if you need a catalytic convertor is the freeze frame data when P0420 was set, the 02 sensor data, for sensor 2 should be above .650 vlt, below that , the catalytic fails