Ecm/pcm issue?
 

I have a problome with my Truck. I keep getting a check engine light p121. Now I know this has to to with the throttle position sensor . So I reset the trouble code i.e. cleared it. It came back on again. So I went to recalibrate it. Disconected my ground on the batteries and let it sit for a few hours. Then reconected the Batteries and turned key on and did the three slow applications of accelerater peddle. It reset. Now my wait to start light isnt working and after a few hours of driving, my truck threw another code. yep, p121. On the way home from work my speedometer started jumping around and finally just fell to zero. The truck seems to be running fine. After reading a few hours on forums, it seems that the Ecm/pcm could be the issue. Does this seem plossible? I had a simular issue a few months back. My wait to start light stopped working and a few weeks later my starter failed, Cheap Napa starter, and after replacing starter and reconecting my batteries, a day or so of disconect, my wait to start light worked again.

A couple things....First what year truck do you have?

Second: on resetting apps you turn the key on and off 3 times leaving it in the on position and then push the peddle slowly all the way to the floor and let it back then turn off the key...all this after letting the truck sit at least 30 mins with the batteries disconnected. You do NOT push the pedal down 3 times.

My truck is a 1999. and I will try the resetting of apps.Thanks, Chris

So, I did the APPS reset and the truck started up and I drove it around the block with none tps code being thrown. Also speedo is working now. When I started the truck the wait to start light worked. But when getting back to house and shutting down and restarting the light didnt work. I tried pushing on cluster and it worked. Maybe cluster conection is dirty or just a huge coincidence!

The cluster conections can be a problem. But try just wiggling the panell first. Sometimes they are a real B**** to plug back in.

With the truck even remotely warm, the wait to start light probably won't come on.
If it's a warm day, mine won't come on but once. (in the morning).

Two intermittent problems showing in the instrument cluster? Sounds like the plug, as you said, or the instrument cluster itself.