So my SRT-6 is showing the P0123 code and going into limp mode when I hit the kickdown switch. I bought another accelerator pedal and have tested the two sensor outputs by hooking up a 5v power supply. The Crossfire Service manual only shows the values for the #1 sensor but not for #2. Is 4.65v too low? BTW, is wide open throttle considered with or without the magic kickdown button engaged?

The used one I bought measures:

#1 Sensor

Pedal At Rest = 0.286v

Touching Kickdown Switch = 4.3v

Kickdown Switch Engaged = 4.62v

#2 Sensor

Pedal At Rest = 0.137v,

Touching Kickdown Switch = 2.14v

Kickdown Switch Engaged = 2.29V

The original pedal out of my car:

#1 Sensor

Pedal At Rest = 0.375v

Touching Kickdown Switch = 4.41v

Kickdown Switch Engaged = 4.65v

#2 Sensor

Pedal At Rest = 0.178v,

Touching Kickdown Switch = 2.20v

Kickdown Switch Engaged = 2.35V

(P0123) APPS SIGNAL 1 CIRCUIT HIGH (CONTINUED) 10. CHECK THE APP SENSOR 1 SWEEP With the ignition on. With the DRB IIIT, monitor the APP 1 voltage. Slowly open the throttle from the idle position to the wide open throttle position. Does voltage start at approximately 0.3 volt and go above 4.7 volts with a smooth transition?
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The problem may not be with the kick down switch. Both the figures you give in your post are very close and it seems to me that if the kick down wasn't working or putting out a proper signal the car would not know you have pushed the peddle down and thus would continue in the current gear without going into limp. Do you have any problems when using the transmission manually or just in drive?
My suggestion would be to get a deeper scan with Mercedes software to access the transmission and engine codes at a deeper level than an ordinary OBDII scanner can do. The generic codes do not give enough info to solve your problem.

 

Is there an OBD reader for Mercedes that doesn't cost an arm and a leg?

I may swap out the throttle body to see if that works. Seems like a lot of the failures with the CPS, TB, APP are due to the semiconductors Bosch uses for the Hall Effect sensors failing or going out of spec.
My car only has about 16K on it.

 

I installed the used accelerator pedal and it's working now. Go figure. I also wiggled the connector on the TB so who knows that may have fixed it too.

 

You are right about those Hall effect switches/sensors. They do not have good longevity.
Glade you seem to have solved your problem. Hope that holds up for you; Enjoy the ride.