1990 Toyota Tercel Trouble Codes

I have a 1990 Toyota Tercel with a trouble code of 71, so I assumed it was the EGR Valve, so I replaced it. The code cleared but 24 hours later the code came back. I asked a sales person at a local parts stores of what could be the problem. They said it is possible that I have carbon build up in the throttle body and it should be cleaned out. So removed the EGR Valve and applied a Seafoam cleaner. I noticed a lot of carbon smoke coming out. But I don't know if I cleaned the throttle body correctly. The code reset. Later in the night as I was driving the code 71 came with with another code of 42. I am puzzled, what do you think could be the problem? Should I replace the Vacuum Modulator Valve and Oxygen sensor or how do I clean out the throttle body correctly and do I remove the EGR Valve again. Also what is a code 42?