2010 Ford Taurus SHO - Low 5V Reference Circuit low voltage was fixed but back

[guitar333]

I could really use some guidance as I'm at my witts end...

I have a 2010 Ford Taurus SHO with a low 5V Reference Signal (P0642).
I first had this issue a couple months ago.
I ruled out the three sensors on this circuit as I still had <1V with all sensors unplugged.
I changed the computer but still had this issue (Doh!).
I then isolated the issue to the engine wiring harness but could not find the fault.
I took it to a shop to see if they could find the issue in the wiring but they couldnt and suggested to change the harness.

After the above, I sourced and replaced the top engine harness last week.
I turned the key on and hallelujah, the reference circuit was reading 5 volts again, it started but ran rough..
It was throwing a code for the MAP sensor and upon inspection I realized the shop replaced the MAP sensor during their diagnosis.
It turns out they installed the wrong MAP sensor (3bar instead of 2bar) so i ordered the correct sensor.

I just received the new MAP sensor, put it in the car and it won't start(!)
I was disheartened to see the 5V circuit was reading low again....this time about 1.8V. Ugh!!

Looking for some feedback or ideas.
For some reason, the fuel pressure sensor is pegged at 5 volts while the 5V Reference signal is low (1.8V) at KOEO.
I am sharing screenshots of the voltage readings before I replaced the harness, the good readings after replacing the harness, and again now.

Looking for any fresh ideas or if anyone can see the correlation between the pegged FP sensor voltage and the 5V Ref circuit.

Thanks for reading this and any suggestions!!




INFO:

Sensors connected to reference voltage A circuit:
Fuel Rail Pressure (FRP)
Manifold Absolute Pressure (MAP/IAT2)
Turbo Boost Pressure (TCBP/CACT)

 

[Lou]

That sounds like I would be tearing my hair out. We mainly have UK members but we have quite a few from across the pond now so hopefully one of them can assist you!