p0420 code 2014 1.0l ecoboost.

Hi 

Last year I had to have my cat replaced due to the front flexi pipe being damaged in an accident. Short while after having it replaced my engine check light came on and threw up this p0420 code. I took it to my garage who put a cleaner in it and reset the light which for around 3-4 weeks cured it until the light came back on. Following this I then took the car back and they decided to swap the cat again under warranty believing it could be a faulty cat. This still never solved the problem and 3-4 weeks later the light came back on so swapped both lamba sensors. Light was off 3 - 4 weeks and then returned again. After having the cat done I suffered a drop in mpgs which since having the sensors done has risen. Every time it reads this p0420 but the car runs smoothly, I even had the emissions tested with the light on and it passes so my garage think its an intermittent problem.

I have also looked at the dtcs on the hidden menu and its showing these codes but don't have a clue what they stand for

C401_64
1607_82
C100_00

Im at my wits end with it so was wondering if anyone could help!

Thank you