Home » GM 6.5 » GM/Detroit 6.5L ENGINE CONTROLS – DTC P0121 APP Sensor Circuit Perfomance

GM/Detroit 6.5L ENGINE CONTROLS – DTC P0121 APP Sensor Circuit Perfomance

Circuit Description
The accelerator pedal position (APP) module provides a voltage signal that changes relative to accelerator pedal position. There are three sensors located within the APP module that are scaled differently.

Conditions for Running the DTC
• The PCM performs this DTC diagnostic continuously.
• The ignition voltage is more than 6.4 volts.
• The engine speed is more than 300 RPM.
• There are no in range faults for the APP 2 sensor or the APP 3 sensor (PCM checks for high and low voltage faults).

Conditions for Setting the DTC
• The difference between the APP 1 sensor and the APP 2 sensor is more than 0.23 volts (PCM compares pre-scaled voltage (internal to PCM)).
• The difference between the APP 1 sensor and the APP 3 sensor is more than 0.50 volts (PCM compares pre-scaled voltage (internal to PCM)).
• All of the diagnostic set conditions met for 2 seconds.

Action Taken When the DTC Sets
• The service throttle soon lamp will not illuminate when one APP DTC sets. The PCM will only illuminate the service throttle soon lamp when multiple APP DTCs set.
• The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Failure Records.

Conditions for Clearing the MIL/DTC
• A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures
• The use of a scan tool.

Diagnostic Aids
There are no driveability issues associated with the APP module un’ess a DTC is present. The most likely causes of this DTC are poor connections or the sensor itself. The least likely is a PCM problem.
An intermittent may be caused by the following:
• Poor connections
• Rubbed through wire insulation
• Broken wire inside the insulation

A scan tool reads APP 1 position in volts. It should read about 0.45 to 0.95 volt with the throttle closed and the ignition ON or at idle. Voltage should increase at a steady rate as the throttle is moved toward Wide Open Throttle (WOT). Also, 90% pedal travel is acceptable for correct APP operation. Scan APP 1 sensor while depressing accelerator pedal with the engine off and the ignition ON. Display should vary from about 0.74 volt when the throttle is closed to over about 3.7 volts when the throttle is held at wide open throttle (WOT). The following chart will check voltages on all APP circuits to see if they fall in normal ranges. The PCM compares pre-scaled voltages (these are voltages that the scan tool cannot read). The scan tool reads only output voltages.

An intermittent may be caused by any of the following conditions:
• A poor connection
• Rubbed through wire insulation
• A broken wire inside the insulation

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.
If a repair is necessary, then refer to Wiring Repairs or Connector Repairs in Wiring Systems.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
4. This step determines if there is a good 5 volt reference.
5. This step will check for an open in the ground circuit.



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