Kevin Smith I just went through the same thing and found this information on how to reset and adjust the voltage to agree with Idle validation I also had P0121 Code I have copied and paste it information for you believe me it works.
"RESETTING THE APPS SENSOR.
(ACCELERATOR PEDAL POSITION SENSOR)
2ND GENERATION 24 VALVE TRUCKS ONLY.
LIST OF TOOLS NEEDED
DIGITAL VOLT METER
# 20 TORQUE BIT SOCKET
# 1 PHILLIPS
# 10 MM SOCKET
Proper service and repair procedures are vital to ensure personal safety of those performing the repairs. Standard safety procedures and precautions should be followed at all times to eliminate the personal injury or improper service, which could damage the vehicle or compromise its safety.
Although this material has been prepared with the intent to provide reliable information, no liability is assumed in reliance of this material.
It has come to my attention that a lot of people think by disconnecting & re-connecting the batteries and stepping on the throttle it is going to reset the apps, no, that is not correct. It is impossible to adjust your apps if you do not know where it is supposed to be set at. That is why you need the voltmeter.
This is the procedure I use to re-set the apps.
With the key on, engine off you need to probe the apps wire to see where your voltage is currently set at.
The best location to probe the wire is on the PCM (power control module) which is located off the passenger side of the firewall.
· You want the C1 connector, this is the connector closest to the engine,
· You want the orange wire with the dark blue tracer which is pin # 23
· Voltage should read somewhere around 0. 5 volts,
· At this point the voltage reading does not matter, it just has to be accurately written down for later reference.
*** TURN THE IGNITION OFF ON THE TRUCK ***
· The apps sensor is located slightly above and a little to the left of the injection pump.
· Remove the black plastic cover that is located by your injection pump.
The two screws that are holding it are plastic, do not put any downward pressure on them or you will never get them off.
· Un do the 6 - 10 mm headed bolts that hold the bracketry in place.
(DO NOT REMOVE THE CABLES)
You will notice the apps is on the back of the bracketry that you just removed, it is held in place by 2 - 20 bit screws. These screws have a little bit of locktite on them so make sure you have a good socket and gently give a little tap with a hammer before attempting to loosen. Be very careful these screws strip very easily and you only get one shot at them.
· On the apps you will find a white tag that gives you the information on what the apps adjustment should be.
· After loosening the screws you can rotate the apps both clockwise and counter clockwise to get the adjustment you need.
· The reading you took at the beginning of the process on pin # 23 should match the white tag on your apps, if not adjust accordingly.
... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ..... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... .
The above information IS what you do.
To clarify it a little I will explain my experience.
With the air cleaner box off you will see the ECM on the firewall. There will be three plugs or connectors and the C1 connector is the closest one to the engine. I unplugged it and found that the pins are numbered. You will be testing the voltage with it connected though. I took and checked continuity between the #23 pin and orange/blue wire and then I knew I had the correct wire. My wire looked orange with black but in any case it is the only orange wire on the C1 connector.
So with the plug in and the key on take your digital meter and secure ground at the battery. Take a needle or safety pin (I have a sharpened piece of music wire) and get it into the orange/blue stripe wire. Then with your meter read the voltage. The voltage you read is coming from the apps/tps. Write this voltage down. As you adjust the apps/tps the voltage will change.
Now to access the apps/tps. It is where stated above. Mine only had one screw attaching the plactic. If you have not removed any of these plastic screws before, here is what to do because sometimes they just turn and do not come out. Take your Phillips and turn the screw. If it doesn't come out slip a small knife blade or tiny screw driver under the head of the screw and assist it in starting to come out. When you get the screw or screws removed, remove the little plastic insert it was screwed into. It will be much easier to replace the cover with those out. After the cover is in place stick the inserts back into the holes after the cover is aligned with the holes on the metal plate and insert the screw.
Ok, now you remove the cover. Just pull it away and up. It should come off easily. You will see a throttle rod, and if you have the automatic here will also be two small cables. The top cable is cruise control and bottom is for the transmission. Leave them connected a stated above. My truck had 5 10mm bolts and one Allen head bolt holding the steel plate in place. I had to use a combination of wrenches to get them out, as there is stuff in the way. I found a 10mm deep socket, and 10mm end wrench and the Allen socket was what worked for me. With the bolts removed and the plate pulled away you will see the apps/tps with the white tag and voltage written on it. Go ahead and rotate the plate 90 degrees so the sensor is up and you have good access to the screws. Where I was confused at first is I thought I had to take a voltage reading at the sensor. Not so. You will just do the adjustment there. The two #20 torx screws are tight as hell. Once you get them broke loose they still turn hard. You won't have to loosen them more than about one turn to get the sensor to move. The amount of movement you will get out of the sensor is not much. It may move 1/4 inch, that's all. I first tried to loosen these screws with a #20 torx screw driver and could not budge them. I took a small hammer and gave them a few taps and then took a #20 torx socket on a short 3/8 drive ratchet and they came loose. Make sure you have a good, tight fitting bit or you may strip the hole in the screw, and then you would just have to put it back together without making the adjustment. Now you have them loose right. Hopefully you can have your digital meter hooked up and can watch it as you turn the senor. To increase voltage turn clockwise. I had to turn clockwise and hold it with pressure as I tighted the screws to get the 0. 441 vdc I was able to get. If I just turned it and tightened the screws I ended up with only 0. 3xx VDC. If you can get the voltage to read the same at the #23 pin wire as the value on the back of the sensor, great. If not do what I did and get it as close as you can.
Put her back together and do your test drive. Remember now that I have not driven my truck enough to know if I was successful in fixing the problem. I do personally know Bill Kondalay of Diesel Transmission Technologies and have faith in the information he offers.
Did your truck throw and codes? Did your check engine light come on? If you got the PO121 and/or PO122 that would indicate a voltage problem between the apps/tpc and ECM. If you have those codes then do the adjustment. If not read the #23 pin, orange/blue wire. If it is a lot lower than 0. 5 volts do the adjustment. If it reads in the 0. 5xx range then the problem may be somewhere else. "
Attention: TDR Forum Junkies 
