My car doesnt accelerate at times it goes 2mph or maybe even slower. i scanned the car and i got the code P2138 . i need help.. what should i do? how do i fix it? sorry im a noob (Cadillac CTS 2004)

Your vehicle has drive - by - wire , electronic throttle control . P2138 Gm Accelerator Pedal Position Sensor 1 2 Correlation

There are videos on youtube on this P2138 APPS Accelerator Pedal Position Sensor Fix 3 5L Chevrolet Impala

P0411 is an air injection pump code
P1125 and P2138 are both accelerator pedal position sensor codes

P1599 - means that the computer detected that the engine stalled when the ignition had not been turned off.
Does your vehicle have a stalling problem?

P2138 - is the Accelerator Pedal Position Sensor 1-2 correlation.
The accelerator pedal has a sensor and the throttle actuator has the one as well. the computer expect thats both sensor will read the same as the accelerator pedal is operated. If one sensor read differently from the other the computer will set this code.

Long story short, you'll have to have shop to check this code out for you since a scan tool will be needed in the diagnosis of this code.

Condition:
Some customers may comment on an intermittent MIL/SES light Illuminated with a message center or an indicator that displays Reduced Engine Power. DTC P2138 - Accelerator Pedal Position (APP) Sensor 1-2 Correlation may be set. The voltage difference between APP sensor 1 and APP sensor 2 exceeds a predetermined value for more than 2 seconds. Refer to DTC P2138 in SI for further diagnostics.

Cause:
On some vehicles, the IP to body harness connector, which carries the APP signal to the ECM/PCM, is located in the left hand kick panel area and if the kick panel is not sealed properly, water enters and causes a short in the APP circuits setting a P2138 code.

Correction:
Important: Do Not replace the accelerator pedal assembly.

Locate the IP to body harness connector, which may be located in and around the left hand kick panel area. Check to see if there is a water leak into the area and repair as necessary.

First of all a scan tool is need for this not a code reader, you need to view scan data after you find what code is stored in the PCM. It could have a problem with the powertrain control module, wiring, or the Accelerator Pedal Position (APP) Sensors. Here are a few codes that could set .
DTC P2120: Accelerator Pedal Position (APP) Sensor 1 Circuit

DTC P2122: Accelerator Pedal Position (APP) Sensor 1 Circuit Low Voltage

DTC P2123: Accelerator Pedal Position (APP) Sensor 1 Circuit High Voltage

DTC P2125: Accelerator Pedal Position (APP) Sensor 2 Circuit

DTC P2127: Accelerator Pedal Position (APP) Sensor 2 Circuit Low Voltage

DTC P2128: Accelerator Pedal Position (APP) Sensor 2 Circuit High Voltage

DTC P2138: Accelerator Pedal Position (APP) Sensor 1-2 Correlation

Here is the definition. You may need to have someone check the accelerator pedal position sensor and the circuit.

P2138 NISSAN Description The Accelerator Pedal Position (APP) sensor is installed on the upper end of the accelerator pedal assembly. The sensor detects the accelerator position and sends a signal to the Engine Control Module (ECM).
Accelerator pedal position sensor has two sensors. These sensors are a kind of potentiometers which transform the accelerator pedal position into output voltage, and emit the voltage signal to the ECM.
In addition, these sensors detect the opening and closing speed of the accelerator pedal and feed the voltage signals to the ECM. The ECM judges the current opening angle of the accelerator pedal from these signals and controls the throttle control motor based on these signals.
Idle position of the accelerator pedal is determined by the ECM receiving the signal from the accelerator pedal position sensor. The ECM uses this signal for the engine operation
such as fuel cut.
Read more: http://engine-codes.com/p2138_nissan.html#ixzz26J3D2x8s

Four seemingly unrelated fault codes;
P0526 - fan speed error
P0141 - downstream O2 sensor lean condition
P2135 - Throttle position sensor fault
P2138 - Accelerator position fault

You need to have a qualified technician diagnose this with a highend scan tool, maybe there is a possibility that a reference voltage is off spec.

P2138 is Throttle Pedal Position Sensor/ Switch D / E Voltage Correlation. Replace the Throttle Pedal Position Sensor/Switch.Remember to have the set code Cleared from the cars computer, this will turn off the check engine light and reset it. Good Luck and have a good XMASS.

Well you might be talking about a cam shaft noise or a crank-shaft noise. How does the oil look?
Is it over-due for an oil change?
What else can you tell me about the cars symptoms?

Is it time for a tune up maybe? Spark plugs, wires, fuel fiter. It's sounds to me like starving for fuel, plugged fuel filter.