Just had the engine replaced, and the engine stall on low RPM

What is your question?

I just replaced the engine with a low milage engine imported from Japan last week. The mechanic delivered the car with the check engine light on, and stated he can figure it out but it could be the knock sensor. After 2 days and total of 10 miles of driving the car would not run meaning it would start, but stalled . I kept my foot on the pedal mostly to get home but when rpm reached 1000 or so it stalled. Took a code reading of 55 and 71. Could you help me fix this because the great mechanic stated NO WARRANTES, so I wonder why!!! Hmmm.