5.9L Cummins owners experiencing a "dead pedal" condition (unresponsive accelerator pedal) most likely have a faulty APPS; accelerator pedal position sensor. This is also sometimes referred to as a TPS (throttle position sensor) since most code readers do not distinguish between an APPS (for diesels) and a TPS (typically for gas engines). Replacing the APPS is a relatively straight forward procedure that will take most mechanically inclined individuals about 30 minutes to complete. When the APPS fails, it is common for the check engine light to illuminate and a DTC P2121 to be set.
The following procedures are specific to the 1998.5 to 2003 24 valve Cummins and select 2004 models.
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• Disconnect both negative battery cables.
• Locate the APPS cover on the front, right of the cylinder head; remove the black cover to expose the sensor.
• The assembly that the APPS is attached to is commonly referred to as the bellcrank assembly. Remove the entire bellcrank assembly; it is held in place by six (6) bolts and requires a 10 mm socket.
• The APPS is attached to the back of the bellcrank assembly. Unplug the electrical connector then remove the sensor from the bellcrank using a T20 Torx driver. The screws are coated in threadlocker and may be difficult to budge - take care not to strip the head.
• With the old sensor removed, install the new APPS. Notice the the slot on the APPS must engage the tang on the bellcrank shaft. Make sure to line up the slot into the tang when installing. If the APPS does not mount flush with the surface, the slot is not installed into the tang properly. When the slot/tang are engaged properly, you'll have to rotate the APPS clockwise to line up the mounting holes.
•The mounting holes on the APPS are slotted so that it may be calibrated, however you'll want to rotate the sensor clockwise as much as possible before tightening.
• Reconnect the sensor connector, then remount the bellcrank assembly.
• Using a digital multimeter, carefully back-probe pin #3 (green wire) of the APPS connector with the positive lead. Connect the negative lead to the negative battery terminal; do not attempt to find a "good ground" elsewhere, an accurate reading is desirable.
• Reconnect both negative battery cables.
• Set the multimeter/voltmeter on a low range, DC voltage setting.
• Start the truck and measure the APPS voltage at idle. Verify that the sensor voltage is between 0.580 and 0.670 at idle.
With your meter on low voltage DC, start the truck. At idle, the voltage should read between 0.580 - 0.670. Locate the idle stop screw and turn it clockwise until you see voltage begin to rise, then back it off half a turn.
• Disconnect the volt/multimeter and reinstall the APPS cover.
• Locate the idle adjustment screw (marked in image at left). Turn the screw clockwise until the voltage begins to rise, then back the screw out one-half (1/2) turn counterclockwise.
• Disconnect the voltmeter/multimeter and reinstall the APPS/bellcrank assembly cover.