Throttle position sensors (TPS) do not usually go bad, but they do need to be replaced as vehicles/engines get older. The vehicle ECU is also very sensitive to voltage variations from this as it affects drivability and fuel mixture.
 
This is a genuine Nissan TPS, part number 22620-35F00.
 
This fit to the following vehicles:
1989-90 S13 240SX KA24E (all models)
1991-1994 S13 240SX KA24DE (all models)
1995 S14 240SX KA24DE (OBD-I models only)
1988-1990 S13 w/ CA18 engine (all models)
 
The TPS has the same plug for the engine harness; this specific part has the connector for the AT kick down switch, but it is not used if you have a factory MT car or have otherwise swapped out the slush box.