Shifting into first before completely stopped.

[prmoulds]

I am a forum newbie, so if I didn’t follow proper forum protocol, I apologize.

 

This is a problem I am having with my 2007 GMC Yukon XL Denali. I am not a mechanic, so any help would be much appreciated.

 

About a month ago, I stopped to get gas and noticed my DTC light was on. I did the ever so worthless OnStar diagnostics and got the standard “take it to the dealer” response. They said it was OK to drive home, so I did. I was only 10 miles from home.

 

I then plugged it into my scanner. It came up with the “P0121” Throttle/Pedal/Position Sensor/Switch A Circuit Range/ Performance code. I searched the forums and people suggested cleaning the throttle body. I cleaned the throttle body, which had quite a bit of carbon build up. I also put Sea Foam in the gas tank. It then, of course, idled high. I found, again, on these wonderful forums; the throttle relearn procedure. Idled perfectly and ran like a champ. The code also did not come back, after I cleared it.

 

The only issue is, it is shifting into first before coming to a complete stop. This causes the truck to jerk right before coming to a complete stop. This is real bad when slowing down to make a sharp turn, since you are still moving and it shifts into first. I took it into the dealer and they stated this was just “normal wear and tear” and nothing unusual. We know our vehicles and I just couldn’t let it rest. I took it home and plugged it back into my scan tool. I looked at the live data and it looked like the throttle readings were maybe off.

 

Could I bother somebody with a 6.2L to plug in their vehicle and post their readings to see if they match? Or let me know if these readings are normal?

 

Here are the reading I got on my truck, idling at temperature:

 

RPM (/min) = 595

TP (%) “Absolute Throttle Position” = 18.8

TP_R(%) “Relative Throttle Position” = 4.3

TP_B(%) “Absolute Throttle Position B” = 19.2

APP_D(%) “Absolute Pedal Position D” = 18.6

APP_E(%)“Absolute Pedal Position E” = 9.4

TAC_PCT(%) “Command Throttle Actuator Control” = 6.7

 

Thanks much for any assistance and/or advice.