I did the little diagnostic, and the code that its giving me is "13". Which refers to the engine coolant monitor. Now for a little background...
i got the light about 2 weeks ago, when I realized that my coolant reservoir was running low. So I refilled and reset the ECU. The car was running great, then tonight I got the light again. We are experiencing much cooler weather than usual down here (Atlanta), so I don't know if that could be a factor. When I tried to start the engine, it stalled, but still I managed to crank it. The radiator is filled up, and so is the reservoir, so I know that it isn't the problem; perhaps it never was.
Do yall have any suggestions? What does the factory manual say about this? Have yall ever had that problem?

Any feedback is appreciated.
Thanks

 

OBD-2 Diagnostic Trouble Codes are always four digits. Your DTC "13" is actually "0103", and it points to the Engine Coolant Temperature Sensor. The factory service manual says this DTC is set when the ECTS signal has an excessively high or low voltage. It suggests testing the ECTS and also the associated electric connector and wiring harness.

On the 4th Generation Maxima engine the Engine Coolant Temperature Sensor is located in the water outlet tube close to the engine end of the upper radiator hose. There are two sensors in that area. The one nearest the hose is for the dashboard temperature gauge. The ECTS, the one you're interested in, is right next to the the gauge sending unit. There is a good picture of these sensors in the Haynes manual on page 3-7.

With the engine cold, disconnect the ECTS and measure its resistance. Reconnect the ECTS, start the engine, run it until fully warmed up. Stop the engine, and repeat the resistance measurement. The "warm" reading should be a much lower value than the "cold" reading.

The specs for the ECTS are:
Engine coolant temperature 68F, ECTS resistance 2.1 - 2.9 Kohms
Engine coolant temperature 194F, ECTS resistance 0.24 - 0.26 Kohms
(Roughly a factor of ten difference.)