Diesel Trouble Codes

P0122 — Throttle/Pedal Position Sensor/Switch A Circuit Low Input (5.9 Cummins Common Rail)

Diagnostic reference for 5.9L Cummins Common Rail 2003-2007 Dodge Ram 2500/3500 ISB. ECM detects pedal position sensor A voltage lower than expected range.

Updated September 2, 2026

What it means

ECM sees APPS Sensor A voltage below expected range. APPS is used on this platform; P0121/P0122/P0221-type faults remain common.123

Most likely causes

  1. APPS sensor A internal failure / worn potentiometer — Pedal assembly Accelerator Pedal Position Sensor (APPS) Sensor A output low or erratic. P0122 is low input; P0121/P0221 family also common on this platform with APPS still used.312
  2. APPS wiring harness chafing / connector corrosion — Open or high resistance in Sensor A signal/return wiring to ECM causes low voltage reading. Typical on Dodge Ram chassis where harness flexes at pedal and firewall.31
  3. ECM reference voltage or ground issue affecting APPS — Low 5 V reference or poor ground to APPS pulls Sensor A voltage low. Verify reference/ground integrity before replacing pedal.3
  4. Pedal mechanical binding / misadjusted APPS — Pedal not returning fully or APPS misaligned can hold Sensor A at low voltage. Inspect pedal travel and mounting.1

How to diagnose

  1. Verify code and freeze frame; confirm P0122 active with key on engine off and engine running.
  2. Inspect APPS connector at pedal and ECM harness for corrosion, moisture, bent pins, and chafing along routing.
  3. Check APPS reference voltage, ground, and Sensor A signal voltage at rest and through full pedal travel with a scan tool live data and a DMM. Compare to expected sweep; low stuck voltage indicates sensor/wiring fault.
  4. Wiggle test harness from pedal to ECM while monitoring APPS voltage to reveal intermittent opens.
  5. If voltages are out of spec, disconnect APPS and measure sensor resistance sweep; replace pedal assembly if non-linear or stuck low.
  6. Clear codes, road test, and confirm APPS voltage tracks smoothly with no low-input recurrence.

Parts and tools you may need

Rough cost and difficulty

Parts cost varies by supplier and labor rates. APPS pedal assembly is the common replacement; wiring repair cost depends on extent of damage.

When to see a pro

If voltage checks pass but code returns, ECM internal input fault or intermittent harness break inside dash harness may be present. Professional diagnosis with wiring diagrams and scope capture is advised. Do not attempt high-pressure fuel system service for this code.

Sources

  1. Curated DTC seed — 5.9 Cummins Common Rail (curated) ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — 5.9 Cummins Common Rail (curated) ↩︎ ↩︎

  3. Curated DTC seed — 5.9 Cummins Common Rail (curated) ↩︎ ↩︎ ↩︎ ↩︎