Martyfa,

There have been issues with the connector on the fan/blower itself. And I think a few have actually had to change the fan/blower/motor itself.
If you don't hear the thing run at cold startup, the issue is not check valves, but the motor itself not running.

If you DO hear it running, well, then - I guess you may be onto something. Its just that I've never heard of anyone resorting to check valves.
But mabye I'm not on here as much as I once was.