CEL Codes

I just recently bought a 1998 nissan maxima. The mechanic who checked it out for me said his scanner could not read and that I may need a new knock sensor. Sure enough the one on there was cracked. So he changed it and gave me tune up. Later I was able to do my emissions and get my tag. I had been having some issues getting it to crank especially in the mornings or after its been sitting for a while. It would drag slow when I tried cranking it or I would have to press the gas pedal to get it to crank. Last week on Monday it finally would not crank at all. I had the whole starter changed last Friday. Right after, my CEL came on! The mechanic tried checking under the hood and clearing the codes that came up. The CEL went off for about 10 minutes but the next time I cranked up it came back on! I had the CEL read at Advanced auto and they gave me codes P0325 (knock sensor 1 ckt malfunction bnk 1 or single sensor) and P0110 (IAT Sensor ckt malfunction). How can I have a knock sensor code now and not before? What did changing the starter have to do with these codes popping up? Could they been there all along? What to do to clear them? Thanks!