PO451 after changing fuel pump

I have a 2009 chevy HHR that I just recently changed the fuel pump in.  The next day after changing the pump I had a Po451 as soon as i started the car.  Threw my Launch scan tool on it and looked at live data on the evap system.  I observed FTP sensor reading of -8.000 mmHG and 2.45v.  This seemed strange to me because all the videos i've seen show their scanners reading vac at about 0 and voltage about 1.5v.  

I performed the purge/seal test with car running and watch the numbers.  It starts at -8 mmHG  2.45v and as it pulls vacuum the -8 will slowly go towards 0 and then upward into the positive all the way to 11.  Voltage will drop from 2.45 and stop right around .6v

My understanding is you should see the vacuum start around 0 and as the vacuum increases go into more negative numbers and the voltage should climb, not decrease.  

Why are my numbers doing the exact opposite?  Is it because it's not an OEM pump or sensor?  Is it the scanner?  I cant get to the ftp sensor without dropping the tank again so i tried to backprobe where the sensor wiring harness plugs in behind the back seat on outside of car.  I can not get any solid voltage readings.  they are all over the place.

If I understand correctly, I should see 5v on one wire, the 2.45v i'm seeing on the scanner on the signal wire, and then the other wire would be ground.

why is the sensor starting at -8 vac and 2.45v?  Why is it not starting at 0 vac (or very close) and 1.5v (GM cars)?  Does this indicate a bad sensor thus the P0451 code?

why is the sensor starting at -8 vac and 2.45v?  Why is it not starting at 0 vac (or very close) and 1.5v (GM cars)?  Does this indicate a bad sensor thus the P0451 code?

A faulty (new) FTP sensor is definitely suspect, IMO. You're absolutely right that the sensor should be reading 1.5V at atmospheric pressure.

Before dropping the tank again, I'd suggest one more test. Key on, engine off, fuel filler cap removed. What does the FTP sensor signal voltage say on the scan tool? If it's still 2.45V, get an OE sensor and do you wiring checks while replacing the sensor.

Maybe an unnecessary test, but it's easy to do, and gives the FTP sensor every excuse to do its job correctly. 

Thank you for the reply.  I did manage to find a very interesting explanation of what's going on, I think anyways!  Have a look at this article and let me know what you think after reading it.  It seems to be very evident that the pressure sensor that came with the fuel pump is not very OEM friendly even though its listed as a direct replacement.  After reading this article I'd have to say the pressure sensor that came with the pump is reading absolute pressure and not guage pressure.  Sounds like GM uses sensors that read gauge pressure which is why they start at zero reading.  

I'm starting at -8 which must be the pressure (not vac) the sensor reads right off the bat.  As i'm pulling vac in the purge/seal test its taking away from the pressure and that's why the negative numbers decrease as well as the voltage.  Totally matches the description of a non-GM (aftermarket) sensor bases off this article, at least, that's my perception.  The polarity's are also different with GM vs others.  Sounds like GM were the only ways that had to be different.  

The good thing is that it's been two days so far since I fixed the kink in my fuel supply line.  Could not find any supporting data that suggested a kinked supply line would cause P0451.  It seems all the data points to vent lines only for that code.  But since I fixed the line i have not gotten the code.  Coincidence maybe, I don't know!  But is the computer satisfied with a pressure sensor that isn't giving it what it wants?  BTW, supply line voltage does not change with fuel cap off.  Yet another reason to stick with OEM parts but like everyone else just trying to keep the car running and save a buck!

For now, I'm just going to drive the car and see if the light comes back on.  If it does then I will have to get a GM sensor and put it in.  Just weird though because the fuel pump that was in the car before this one was aftermarket as well and I did not have this problem.  Technically the sensor is functioning properly it's just not to GM specs.  That's probably what's throwing the code.

Here is the link to the article
gotech.com/resources/db/evap-vacuum-pressure-sheets