2013 Nissan Sentra. Why does my car crank but will not start?

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I have a 2013 Nissan Sentra. The car cranks but will not start. I took it to the mechanic, and he said it was code B1550. He said I would need to get a mobile technician because it was related to the keyless / Fob system. so I towed it back to my house. Charged up the car battery, and changed the battery in the fob. Checked the fuses and battery connections. The car started up and ended up running like there was no problem for a few days. Now again it cut off and will not start.