Suppose a commercial product uses a FCC pre-certified WiFi module. If the quality is not consistent throughout the batch, I'd guess some pre-certified modules may produce unacceptable emissions. In that case, could these emissions fail the unintentional emission test for the final product? Or is it that pre-certified modules are somehow isolated during the test such that the emissions produced by the pre-certified modules will not affect the unintentional emission test outcomes?
Well possible that an unit that has approved building blocks fails as one new system. The cumulated total emission can be too much due a simple summing or due increased individual emissions caused by new interactions. Do not rely only on "all my parts already have the certificate".
Do not believe? What interactions????
Think about a radio device and a too high voltage power supply for it. The higher voltage can easily raise the spurious outputs too high or the output at the legitimate frequency can have too high power.