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I own a 2017 Fusion SE 2.0 and I am trying to troubleshoot a P0451 EVAP System Pressure Sensor/Switch A Circuit Range/Performance fault. The fault only shows up intermittently, when the fuel level is below 30%.

Filling the tank clears the check engine light, and it can be several months before the same code shows up again.

I took about a minute of data from the Powertrain Control Module while idling. The data just seems totally erratic...

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Would you diagnose a faulty fuel tank pressure sensor?
 

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That is interesting. Especially the part where the warning only comes on with the tank at 30% or less.

I have attached a diagram of the system.

Is the graph you posted of voltage???

The next time it comes on leave the engine running and remove the fuel filler cap.
Any rush of air into or out of the tank?
Does the light turn off?
Does ambient temp make a difference?

Is the fuel filler cap seal an issue where it meets the fuel filler neck?
Have you been under the vehicle inspecting the wiring and lines?

The code is usually for a bad pressure sensor or the wiring. But that does not explain why the light is on at certain fuel levels in the tank.
A clogged tank relief could be an issue as the fuel is being used.

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