what does code P0420 mean

P0420 - Catalyst System Efficiency Below Threshold Bank 1 -- As the code description implies the P0420 code means that the vehicle's control module has detected that the three-way catalytic converter is not working properly (is not as efficient as the factory is expecting). Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases the catalytic convert needs to be replaced to fix the problem. If the P0420 code is combined with other codes, try fixing the other codes first.

probley be the front lamda sensor

tenspeed is right...sounds like a bad 02 sensor...affects emissions.