P0171 Ford -

Question

2003 sport trac replaced injectors ran good a couple of weeks. Same code now with a hissing type sound
thinking bad injectors: or not sealed

Answer 1

P0171 Fuel trim lean

Vacuum leak , the cause for this code condition is vacuum this leak  this is the hissing noise , one the hissing noise is found and fix the problem is solved, Here is a Ford with a hissing noise, the use of brake cleaner or carburator cleaner help find the problem and in this case it was a disconnected vacuum hose