V.W.
I honestly don't know.
What I do know is that our lawnmower repair guy has told my husband that self-propelled models (for whatever reason) tend to need repair work more than the standard push variety. Husband always wants the self-propelled ones (I can understand why! I was out mowing the lawn earlier today...) and even with annual tune-ups, etc... we still end up replacing our mower about every 4 years or so. On average.
The last time I looked, Toro did not have the best ratings. It seems like at one time they were a great brand, but not so much anymore. $400 is not an especially expensive amount for a self-propelled. Especially if you got the large rear wheels.
I think, as aggravating as it sounds, that if Home Depot is willing to repair it, that I would take it to them and let them repair it. If they CAN'T repair it, then I would be looking for some sort of "lemon" replacement discount. A HUGE one. And I'd choose a different brand.