I have a 1997 lexus es300 check engine light came on code PO420 what could this been

Code P0420 indicates cat convertoer efficiency below threshold. Sorry, but this usually means the cat is shot. Probably will need pro diagnosis, and if cat is indeed bad, have them try to figure out what killed it. Can be anything from misfires, to coolant getting in the combustion chamber. Outside chance of o2 sensor problem, but this one usually requires the sensors to be working correctly to set. Due to the diagnostics neccessary, probably you will save money by having it looked after by an experienced independant, who's familiar with this model.