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.

 

any dead pedal or power loss while driving with the code active?

 

Before you call it bad computer you should find a qualified tech to diagnose the truck. No reason to throw parts at a problem and there is absolutely no way anyone on a forum can tell you of its bad. You would have to do atleast an hour of extensive testing.

Sent from my DROID RAZR using Tapatalk 2

 

Have you tried readjusting your throttle position sensor? Scan tool would come in handy for this or you can do it with a dvom.

 

I've found that a scan tool and dvom don't offer a high enough refresh rate to see glitches in tps circuits. You could look at it for hours and not see the glitch that you could see with a DSO. Of course powers and grounds should be checked before replacing apps. And I've found that regrounding the pcm helps quite a bit with issues like this.

Sent from my DROID RAZR using Tapatalk 2

 

I don't think the apps is necessarily bad. Maybe just worn. If the code is p0121, that's showing a volts mismatch with idle validation switch. Wouldn't actually be looking for sweep values just at rest value to start with then go from there. It would be nice to see what the voltage is with pedal at rest to see if it is above or below spec.

 

Right, but if you look at when those pids are monitored you'll see my reasoning for watching posistion values as well.


Sent from my DROID RAZR using Tapatalk 2

 

I completely agree with you on watching the sweep and doing so with a scope. Im just saying start by a quick check and go from there.