Have Ford Questions? Ask a Mechanic for Answers ASAP
P1131 is one those dreaded Ford-specific codes; this one in particular sets when the system is trying to compensate for an overly lean condition. Possible causes range from a partially stuck-open EGR valve, to intake and/or exhaust manifold leaks, to faulty PCV system, low fuel pressure, to even a dirty MAF sensor.
We've always started our diagnosis by a thorough visual inspection of the intake and exhaust systems; next, we plug in the scanner and monitor EGR and MAF PID to ensure the valve is not open when it's not supposed to and the MAF is measuring air intake correctly; from there we measure fuel pressure to ensure it is up to the 35-45 psi spec. Once that's been checked, we move to inducing rich and lean conditions to confirm the sensor itself is good. Once we know it is, then we erase the code and put the vehicle through a Drive Cycle after which we look at Mode $06 for information on what monitor failed and why to determine what is making the PCM set the code.
I've seen independent mechanics guess at the problem and replace many expensive parts such as O2 sensors, EGR valves, fuel pumps and MAF sensors only to have the code return.
I can guide you through all the diagnostic and repair procedures that are sure to lead us to the root cause of the issue, but please be aware that there is a 99.9% chance you might need specialized tools and equipment to perform some of those procedures.