My Dodge Ram Van came up with the engine light today and I went to Advance Auto and had a diagnostic ran. The code came up PO121. My trans fluid was a little low as well. I filled the fluid with transmission fix and the light went out.....seems to be running alright now, but I'm worried about it. Has anyone else had this problem? Does it come back? Or is it gone for good...?

 

I will start out by saying that it is entirely possible for a TPS to set a code if you run with low transmission fluid. However, that fluid would have to be low enough to be causing you some drivability problems, such as jerky or late shifting, etc.

Once you have filled the transmission drive it and wait and see if the problem recurrs. It may not. Do not waste any money on a new TPS or trying to clean the old one until you know for sure.

Should the problem come back, then the following will be of some help for you. The 121 means that the voltage being sent from the TPS to the ECM is out of range for what the ECM expects relative to the engine speed and transmission speed sensor or governor feedback (depending on model and year of your Dodge). The TPS being either too high or too low is a common problem after some age on a rig. The least expensive solution is to clean the TPS and see if the problem will go away. Otherwise, you will need to change the TPS. There is an excellent article on this site about how to clean the TPS.