Hey! I got a 2004 Nissan Sentra with the 1.8L and I'm getting code P1273 which is the first oxygen sensor reading the car is running to lean. There is two oxygen sensors I already replaced the first one at the manifold with a original brand which is Bosch which Nissan uses and I had a different code for just this sensor but now I have this code. I'm thinking it's fuel pressure because sometimes the car would crank but won't start but after you wait a few seconds with the key still on the car could hesitate and shake for a few seconds and start and run perfect on the second or third attempt to start it. There is also a freeze frame showing what looks like the car coming to a stop for a stop sign and running lean at that time. I also rented a fuel pressure testing kit at AutoZone and found out when you turn the key without starting the engine fuel pressure would jump to around 30 psi and then drop down to close to zero just in about 1 minute or less. Then if you start the engine the same thing happens as I mentioned above with the engine starting but with the engine on the fuel pressure is close enough to around 50 psi and the normal fuel pressure for this car is around 52 psi according to the internet. This Sentra has the electric fuel pressure regulator inside the fuel pump assembly and this one does not have a return system. Older ones I know have vacuum controlled fuel pressure regulator and have a return system. I've searched and people have a lot complains about the fuel pumps and regulators going bad all the time in these model cars. I've already tested for vacuum leaks and found none and the mass air flow sensor was recently cleaned and is working correctly. Cleaner was also run through the system too. Thanks!