The old combine on the farm had not been operating in almost a decade, but after a few hours of work, it was operating flawlessly in the field.
on the farm -- prep phrase (adjective)
in almost a decade -- prep phrase (adverb)
after a few hours of work -- prep phrase (adverb)
in the field -- prep phrase (adverb)
So there are 4 prepositional phrases in this sentence.
