Key on only, the TPS Reading will toggle from 0.0 to 0.4. At full opening it reads 100.0 and will snap back to 0.4 when pedal released.

Does this mean the tps is bad or is this a normal sort of thing

 

year of veh, what veh in converstaion and what engine?

 

1999 4.3 vin X vortec

 

What are you using to measure the TPS? Units of measure (volts?)?

To test, you must back probe the dark blue wire (position C) in the TPS connector (with the connector connected to the TPS) and then ground out your multimeter ground wire, then monitor the voltage on this wire with the key-on, engine-off. A properly functioning TPS should read around 0.4V with the throttle closed and around 4.5V at wide open throttle. If you get voltage drops while moving between open and closed, then the TPS sensor is bad. If it varies while closed or while open, then I would suspect a wiring fault between the PCM and the TPS.