05 Equinox engine light came on had scanned codes were p0455&p0128 what do I do? on 2005 Chevrolet Equinox

thought it may be gas cap, went and bought a new capand the same day the light went off. 2 days later the light came back on. car is running fine, there are no new noises come from it either. I have to get echecked in 8 days or less.... Help!!!