Hello, I will try to help you, although I'm in the UK also so I am relying on research and data.
What is the actual issue, i'm guessing a non start but it would be good to clarify and what have you tried so far.
Code P0251 would usually be stored if there is a fault related to the pump optical sensor.
The engine would still run with a failed sensor, albeit in reduced power mode.
When it comes to the optical sensor and the DTC P0251, one possible cause is the optical sensor filter harness.
These are well known to fail. Follow the harness coming out of the optical sensor and the filter harness will look like a little box added in to the wiring, the plugs match up so just unplug it and plug straight to the sensor.
As you're question specifically was about how to test the optical sensor, I have obtained you the information on how to troubleshoot the dtc P0251, like you asked, I also do try to be helpful and am happy to continue a little further usually than the specified question of it helps.
Let me know whats what and if the procedure helps, if the engine doesn't run, on the Hz test you won't reach 900 rpm or 60 hz, I would expect to see 30 or 40hz at cranking, I think it referred to j and a number, a multimeter that can measure frequency will do.