I replaced my TPS about 4 months ago. In the last week I changed my transmission fluid and filter. Truck shifts much smoother but now the check engine light is on indicating P0121. It is saying Throttle Position Sensor. Is it possible the TPS is bad already?

 

If the truck shifts and runs OK and hold steady speed like normal then the TPS is probably OK. You could just have a stored TPS code that has showed back up at random for some unknown reason. Maybe try a hard reset on the PCM and see if the code comes back. You can reset the PCM by disconnecting the negative battery cable, hold the ignition key in the start position for about 30 seconds and then re-connect the negative battery cable. Or you can stop by a parts store and see if they will use a code reader to clear the code for you.

Jimmy

 

The only thing that I noticed was yesterday I started the truck ir revved high for a few seconds until I kicked the throttle and it slowed the idle. Never did that before.