E 74 code pops up when i turn on my sons xbox

E74: AV cable error... There is a problem with the AV cable, try using a different AV cable. If the cable is known to be working then there is a 90% chance it's a scaler chip problem (the "ANA" or "HANA" chip connected directly to the AV cable) in rare cases it is the GPU.

It is also related with the cooling system of GPU, if still unresolved on the first solution, you should call MS if you are still within warranty..