Diagnostic Trouble Codes.
Undo the battery negative cable for approx 20 minutes. That should remove all codes and trouble codes. They will however return if its a trouble code not diagnostic, if that makes any sense.
DTC stands for Diagnostic Trouble Code.
Trouble code PO151
In a 1988 Nissan D21, the diagnostic connector is typically located under the dashboard on the driver's side, near the steering column. It may be housed in a plastic cover or be exposed, and is often identified by a rectangular shape with multiple pins. This connector is used for accessing diagnostic trouble codes from the vehicle’s onboard computer.
Take it to Auto Zone and they will read them free.
fuel temperature sensor a circuit
The diagnostic port on a 2013 Nissan Altima is located under the dashboard, near the driver's side. Specifically, it can be found just above the brake pedal, behind a small cover. You may need to crouch down or use a flashlight for better visibility when accessing it. This port is used for connecting OBD-II scanners to retrieve diagnostic trouble codes.
to find out who messed it up0-
Auto Zone will check your computer codes for FREE...
Trouble code P0340 means:Camshaft position sensor circuit malfunction
Code 12 just means that the ECM is going into diagnostic mode, this is NOT a trouble code! Code 12 should blink a couple of times pause then blink your trouble codes if you have any. If you have no trouble codes, it will resume to blink code 12 again.