Actually, I DO think you'd have to pay labor on a bad part. It's not the contractor's fault that the part went bad... it's the manufacturer's. And the manufacturer won't reimburse the contractor for his time. If the contractor had to provide free labor for anything that went wrong with a product he doesn't make himself... that would be a lot of risk for him to assume.
I had something similar happen. I had new windows installed several years ago and they fogged up due to the altitude and intensity of the sun. It was a warranty issue and the manufacturer replaced the windows at no cost. But I still had to pay the installer to take the old ones out and put the new ones in.