Nissan frontier 2006 will start but dies immediately. Sometimes it will start but more often not.

Honestly, it could be any number of things ... When it cranks, does it seem to turn over slower than usual ? If so, I'm thinking your charging system is not providing adequate voltage to the ECM to keep it turned on when you start the vehicle. It could also be the ECM relay, fuel delivery ( fuel pump or fuel pump electrical circuit ), or any number of sensors on the engine. When the engine DOES start and stay running, does it ever seem to "stumble" or "hiccup" while it is idling ( like it is going to shut off but keeps running ) ? If this happens, then I am thinking along the line of a failed ( or failing ) engine position sensor like the Camshaft position sensor ( CMP ) or Crankshaft position sensor ( CKP ). If you are absolutely positive that the gas in your tank is good and you are getting gas delivery at engine start and, you do not have a charging system or battery issue, I would be thinking about those two engine position sensors as possible culprits. Not sure where you are located but two auto parts stores that I know of -- Auto Zone and Advance Auto offer FREE diagnostic fault code reading ( you obviously have to get the vehicle to the store though ) which would give you a starting point at what may be causing your issue and, by the way, a failing CMP or CKP WOULD leave a fault code but, if the vehicle is running, the fault would be inactive. Hope this helps -- Good Luck !

Mechanic can't figure it out. Throws up no codes on computer. Next stop Nissan dealership.

get a con of carb cleaner and spray the throttle body clean, i had a similar problem Nissan changed the computer and some more , it cost me over $2400.00 . The prob was still there. I sprayed the throttle body while engine ran and then again and let it sit over night. Problem solved.