Nissan Repair: 97 pathfinder, oil pressure switch, mass airflow sensor

Question
Just purchased a 97 pathfinder 4wd. Seemed to run fine for the first month. However, decided to do an oil change and replace spark plugs. Almost immediately afterwards, the truck started having problems starting and hesitated while driving. BF started it up and let it run for about 10min, turned it off because he forgot something, then it wouldn't start back up. I started it about a half hour later and it started right up and I drove it to work with no issues except the hesitation and sputtering a little bit. The next day he started on his way to work and saw smoke coming from the hood. Took it to a mechanic that wknd and was told it was the knock sensor. Replaced that and now when the mechanic tries to start it, it only stays on as long as the ignition is being engaged by the key. So, I guess basically its not starting or staying started. How many issues could this possibly be?

Answer
Danya,

It could be as simple as the oil pressure switch being defective.  If the oil pressure switch is defective it will turn the fuel pump off after you release the key.  A safety feature if the vehicle turns over and there is no oil pressure the fule pump will stop.  I would also have the mechanic do an ohms check on the coil to make sure it is good and be sure to have the ignition fuse checked to make sure it is good.  So, basically, have the fuel pressure checked when it is started.  The computer codes should be read again to make sure there are none stored in memory.  Another cause could be the mass airflow sensor going out so the mechanic can ohms check that too.  The mass airflow sensor will not throw a code most of the time it goes out so the only way to know for sure is to perform an ohms check.