A "lemon" usually refers to a car that has probles right from the start. This can refer to any product, meaning the same thing. A product that has hidden defects that come out later on, no matter how often you take it in to get it repaired, either the same, or another problem will come along.

So a "no lemon guarantee" means that the customer is guaranteed not to have to keep the same product if it requires a third repair. It will be replaced.

a "lemon" is a product that is fundamental defective in some way - it's never going to work properly.

If you have been sold a "lemon", then they will replace it with a new one. A "lemon" is often defined (according to hits found on google) as a product which has undergone 3 repairs and *still* doesn't work properly -> fundamentally defective -> replace it.

I don't know what is the context, but "lemon" in colloquial English is a machine that is defective, or prone to break apart.
"This car is lemon", means the car is not well built.
Likewise, no-lemon guarantee means "I guarantee that this car is reliable. So much so that I will replace it with another car at 3rd repair." The seller is emphasizing the car is so good the 3rd repair is unnecessary.
M-W dic. definition of "lemon" is as follows:

one (as an automobile) that is unsatisfactory or defective