Check engine light is on. Had it put on Computer to check and states, cont on 2000 Nissan Altima

its either EGR valve or catylic converter. Please explain what to do first here. Thanks, Im very handy with repairing vehicles. Cheapest first here please.

Asked by for the 2000 Nissan Altima
rec. fuel system clean. ethanol treatment to 1 tank premium. ethanol blend fuel causes build up in fuel system, combustion chambers, exhaust and causes increased wear to spark plugs. this build up can trip your sensors.
treatment will take time. disconnect battery after tank finished to reset sensors.