If the obd plug is really gone there is another way to check for the DTC codes.
Nissan Vehicles Engine OBDII Codes Read Without Nissan CONSULT or Scan Tool
The Engine Control Moduel (ECM) has an on board diagnostic system, which detects malfunctions related to engine sensors or actuators.
The ECM also records various emission-related diagnostic information
The malfunction indicator lamp (MIL) on the instrument panel lights up when the same malfunction is detected in two consecutive trips (Two trip detection logic), or when the ECM enters fail-safe mode.
Instructions
The first step to read the codes is setting the ECM in Diagnostic Test Mode II. Once the ECM is in Diagnostic Test Mode II, the "Service Engine Soon" warning light will start flashing. Count to flashes to determine the code
A particular trouble code can be identified by the number of four-digit numeral flashes. The 'zero' is indicated by the number of ten flashes. The length of time the 1,000th-digit numeral flashes on and off is 1.2 seconds consisting of an ON (0.6-second) - OFF (0.6-second) cycle. The 100th-digit numeral and lower digit numerals consist of a 0.3-second ON and 0.3-second OFF cycle. A change from one digit numeral to another occurs at an interval of 1.0-second OFF. In other words, the later numeral appears on the display 1.3 seconds after the former numeral has disappeared. A change from one trouble code to another occurs at an interval of 1.8-second OFF. In this way, all the detected malfunctions are classified by their DTC numbers. The DTC 0000 refers to no malfunction.
How to read the engine codes without a Nissan CONSULT or scan tool. (Set ECM in Diagnostic Test Mode II)
1Confirm that accelerator pedal is fully released, turn ignition switch ON and wait 3 seconds.
2Repeat the following procedure quickly five times within 5 seconds.
aFully depress the accelerator pedal.
bFully release the accelerator pedal.
3Wait 7 seconds, fully depress the accelerator pedal and keep it for approx. 10 seconds until the MIL starts blinking.
4Fully release the accelerator pedal.
5ECM has entered to Diagnostic Test Mode II (Self-diagnostic results).
How to clear the engine codes without a Nissan CONSULT or scan tool.
1Set ECM in Diagnostic Test Mode II (Self-diagnostic results).
2Fully depress the accelerator pedal and keep it for more than 10 seconds. The emission-related diagnostic information has been erased from the backup memory in the ECM.
3Fully release the accelerator pedal, and confirm the DTC 0000 is displayed.
Note:If the battery is disconnected, the DTC will be lost from the backup memory within 24 hours.
Note:Be careful not to erase the stored memory before starting trouble diagnoses.
If you don't understand the above go to
http://www.brianhaddock.com/2010/reading-the-engine-diagnostic-codes-on-a-nissan-altima