Is it possible to disable enngine management light?

I bought a car ford fiesta Lx 2004 1 month ago with 3 month warranty, within a week engine manage ment light came on while driving, i took into the dealer there was problem with O2 sensor and catalytic converter convertion rate too low,dealer change that O2 sensor then he told me sensor was the problem that is the reason catalytic convertion rate showing low. After a week again engine management light came on then i left my car with dealer 2days to diagonise the fault after words never engine management light came on.But when i bought that car engine management light come on when it starting and extinguish when engine on only lights came on running condition when fault with engine,After that last maintenance never light came on starting process as well as when engine had problem i try to contact that dealer he saying he don't know any thing, I took into another person for fault scanning it showing fault p 0420(catalytic conversion rate too low). If you dont mind any body can tell me is possible to disable engine management light through software, all other lights in dash board working as per manual showing and car running smoothly exept smell from exhaust.please suggest me any short cut for cleaning catalytic converter.