Question

Code P1391 what is it and how to fix glow plug input circuit low voltage on my 2004 Ford?

Answer 1

The engine code P1391 indicates a problem with the Glow Plug circuit of Bank 1. You could start by checking the glow plug wire connectors and harness for damage. Here is a link to the code page for more information:

P1391 Ford Code - Glow Plug Circuit Low Input Bank 1
Read more: https://www.autocodes.com/p1391_ford.html