I would like to test the throttle position sensor on my 1985 300zx, where is this located. I have been having trouble starting the car and keeping it running once it is started. Also when gas pedal is not engaged it somtimes seems like the throttle is stuck. Also car is somtimes bucking when driving.

Your 2000 Frontier only has the one TPS sensor.

first off P2 codes are factory codes so googling those gets lies easy. only the shop manual VW is the right way and it even tells you step by step how to prove what is wrong, (tests)
country not told , engine yes, VW yes ,body yes. stick shift yes.
USA answers this web site no COUNTRY stated so must be told.
yes TBW cars the TB and pedal pots all must match or it goes to limphome, like all modern cars do i fact.
from the book (USA)it states clearly this.

Number of Trips to Set Code: 1
Trouble Code Conditions:
Engine started, battery voltage at least 11.5v, all electrical components off, ground connections between engine and chassis well connected, the ECM detected that the accelerator pedal position sensor signal was outside the parameters to function normally. Note: Both the Throttle Position (TP) Sensor and Accelerator Pedal Position Sensor are located at the accelerator pedal module and communicate the driver's intentions to the ECM completely independently of each other. Both sensors are stored in one housing.
Possible Causes:

• * Ground between engine and chassis may be broken
• * Throttle position sensor may have failed
• * Accelerator Pedal Position Sensor has failed
• * Throttle position sensor wiring may have shorted
• * Throttle position sensor has failed
• * Faulty voltage supply
• * ECM has failed <<<<(nah I say)

Trouble Code: P2128 (1.8L L4 VIN - Std)
Accelerator Pedal Position Sensor 'E' Circuit High Input

now learn this on TBW, throttle by wire,
you must do a learn cycle to swap parts for sure not willlyynillyyy
the TB and the pedal modules are married and turned
to like each other, and learm mode step is next. changing many parts, for sure all that guessing and throwing parts are hopeless.
Throttle Valve Control Module J338 Throttle Drive (for Electronic Power Control (EPC)) G186/Throttle Drive Angle Sensor 1 (for throttle drive) G187/Throttle Drive Angle Sensor 2 (for throttle drive). G188 6-pin electrical harness connector 2.
those are the parts.
I see no learn mode.
Final procedures

After the Repair work, the following work steps must be performed in the following sequence:

Check the DTC memory. Refer to Diagnostic Mode 03 - Read DTC Memory.
If necessary, erase the DTC memory. Refer to Diagnostic Mode 04 - Erase DTC Memory.
If the DTC memory was erased, generate readiness code. Refer to Readiness Code.

End of diagnosis.
the tests in them manual are long for both devices TB/pedal

reset All DTC errors at parts swaps and
key off
then pull battery NEG log for 1 or more hours.
then put back and tranny will self learn
and any others, told in the service manual

Next time ask what to do first, on modern cars.
Id have swapped the pedal parts all first.
reset all DTC
and pulled battery neg lug.
and waited, and put back battery and drive so PCM can learn the new PEDAL values, and also the TB (paired to it)
if that is too costly?
the manual shows the tests on each pint here Pedal and voltages and ohms to find the exact cause of the error, if lucky
do learn this, most go intermittence, and super hard to prove that.
best is replace the pedal POTS in whole.
if TB fails it too in whole and never buy used parts here.
cardone1 may have it but great quality rebuilt.

throttle position sensor is located on throttle body, ad is usually replaced when bad.


https://www.yourmechanic.com/article/how-to-replace-a-throttle-position-sensor-by-mia-bevacqua

http://www.hemanual.org/throttle_position_sensor_tps_schematic_diagrams-2420.html

https://www.yourmechanic.com/article/p0700-obd-ii-trouble-code-transmission-control-system-malfunction-by-spencer-clayton

https://www.yourmechanic.com/article/p0707-obd-ii-trouble-code-transmission-range-sensor-circuit-low-input-by-spencer-clayton

Throttle position sensor
A throttle position sensor (TPS) is a sensor used to monitor the throttle position of a car. The sensor is usually located on the butterfly spindle/shaft so that it can directly monitor the position of the throttle. More advanced forms of the sensor are also used, for example an extra closed throttle position sensor (CTPS) may be employed to indicate that the throttle is completely closed.
https://www.youtube.com/watch?v=_skVHdgtMTU

these links should assist GM OBD Trouble Code 22 How to test TPS sensor on any GM throttle position sensor GM OBD Trouble Code 33 Rough idle MAP code 33 intermittent no start shorted injectors GM have code 33 have replaced the MAP sensor vacuum looks

P1121 - Throttle Position (TP) Sensor Inconsistent with MAF Sensor The PCM monitors a vehicle operation rationality check by comparing sensed throttle position to mass air flow readings. If during key ON engine running self-test the comparison of the TP sensor and MAF sensor readings are not consistent with calibrated load values, the test fails and a diagnostic trouble code is stored in continuous memory.

• Air leak between MAF sensor and throttle body
• TP sensor not seated properly
• Damaged TP sensor
• Damaged MAF sensor

Drive vehicle and exercise throttle and TP sensor in all gears. A TP PID (TP V PID) less than 4.82 % (0.24 volt) with a LOAD PID greater than 55% or a TP V PID greater than 49.05% (2.44 volts) with a LOAD PID less than 30% indicates a hard fault.
Do you know what scan data on a scan tool is ?
How to Install Replace Throttle Position Sensor TPS 2001 Ford Ranger Buy...

P0122 is TPS signal low, which could mean you have a bad TPS, or a fault in the wiring or connector. I suspect a bad Throttle Position Sensor.

It is at the throttle pedal.

How to Replace a Throttle Position Sensor
(DIY & Save up to $135.75 ) - Time: 2.1 hours

Parts:
1. Throttle Position Sensor - $ 55.44

Tools:
1. Scan Tool
2. Screwdriver
3. Multimeter

Procedure:
1. Locate the OBD data link connector (DLC). Refer to your owner’s manual for the specific location.

• Tip: The OBD-I data link connector is usually located in the engine compartment near the battery or the fuse box. The OBD-II connector (post 1995 cars) is usually located beneath the instument panel next to the steering column.

2. Turn your key to the“ON” position. Do not start the engine.

3. Plug your scan tool into the OBD data link connector. Turn on the scan tool and select the appropriate steps to read the diagnostic trouble code (DTC).

Scanner plugged into OBD-II data link connector.

• Tip: The OBD-II diagnostic trouble code (DTC) is a 5 character alphanumeric code. The first digit is a letter that indicates the component group, the second digit is OBD mandated or manufacturer specfic, the third digit is a subgroup, and the fourth and fifth digits define the component and basic problem.
  

• 4. Locate the throttle position sensor.

• 
  
• Throttle position sensor
  

• 5. Disconnect the electrical connector from the throttle position sensor.

• 
  
• Electrical connector removed
  

6. Test the resistance of the throttle position sensor (TPS) with a digital multi-meter. If the resistance is out of specifications, the TPS must be replaced.

• 7. Remove the throttle position sensor.



Removing sensor mounting screws

8. Install the new throttle position sensor. Tighten the mounting screws and attach the electrical connector.

• 9. Erase the diagnostic trouble code (DTC) with the scan tool using the“delete” or“erase” command. Road test your vehicle to see if the check engine light returns.

Check engine light


• Tip: If the check engine light returns, repeat the diagnostic process for additional DTCs or take your vehicle to a certified repair shop.

Direct OE replacement

Hope helped with this. Good luck (remember rated this help).

THIS ENGINE HAS ELECTRONIC THROTTLE CONTROL..THERE ARE 2 THROTTLE POSITION SENSORS LOCATED IN THE THROTTLE BODY,COMPLETE ASSEMBLY MUST BE REPLACED IF DEFECTIVE..IT ALSO HAS SENSORS IN THROTTLE PEDAL ASSEMBLY AS WELL..WHAT TROUBLE ARE YOU HAVING?