Diesel Trouble Codes

P2121 — Throttle/Pedal Position Sensor/Switch D Circuit Range/Performance — 5.9 Cummins Common Rail

Diagnostic reference for 5.9L Cummins Common Rail 2003-2007 Dodge Ram 2500/3500.

Updated October 15, 2026

What it means

The pedal position sensor D signal is outside its expected range or does not track the other track. On the 2003-2007 Ram the electronic APPS at the pedal is a common failure; expect idle-only or low power.1

Most likely causes

  1. Accelerator pedal position sensor (APPS) internal wear or electrical failure — The APPS is integral to the pedal assembly and is a common failure point on this platform. Internal wear can cause the D track to drift out of range or lose correlation with the other tracks.1234
  2. Corrosion or loose connection at the APPS connector or in the under-hood harness — Moisture and heat cycling can degrade the pedal sensor connector, leading to intermittent resistance or open on the D circuit.23
  3. Chafed or damaged wiring between the APPS and the PCM — Wiring that is rubbed through or has poor crimps can introduce noise or voltage drop on the D signal, causing range/performance faults.23
  4. PCM connector contamination or poor terminal contact — Corrosion or debris at the PCM plugs can affect the APPS signal circuits and set a range/performance code.2
  5. Mechanical binding of the accelerator pedal/APPS pivot — Binding at the pedal hinge can cause erratic sensor output and non-linear pedal travel, leading to range/performance detection.45

How to diagnose

  1. Verify the code is present and review freeze-frame for pedal position, vehicle speed, and engine load at set.
  2. Inspect the APPS connector at the pedal for corrosion, bent pins, and secure latch.
  3. Perform a visual and tactile inspection of the wiring harness from the pedal to the firewall and to the PCM for chafing, heat damage, or previous repairs.
  4. With key on engine off, monitor APPS voltage tracks with a scan tool while slowly sweeping the pedal from idle to full throttle; look for dropouts, flat spots, or divergence between tracks.
  5. Check reference voltage and ground supplies to the APPS per the service information; confirm stable supply during pedal sweep.
  6. Inspect the accelerator pedal mechanical pivot for binding, debris, or wear that could affect sensor linearity.
  7. If the APPS signal is erratic or out of range, compare to a known-good sensor or substitute test pedal assembly to isolate sensor vs wiring/PCM.
  8. Clear codes, perform a road test with live data logging of APPS tracks to confirm the fault does not return under normal driving conditions.

Parts and tools you may need

Tools you’ll need

Rough cost and difficulty

Likely repairPartsShop laborWho usually does it
Wiring or connector repair$10–$1001–4 hrsDIY with tools and a manual
Accelerator pedal position sensor$80–$3000.3–1 hrsEasy DIY

Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.

When to see a pro

If the ETC light flashes and the vehicle enters limp mode, or if you experience unintended acceleration or loss of throttle response, stop driving and have the vehicle towed to a qualified diesel technician. Do not attempt repairs on high-pressure fuel or other safety-critical systems. Professional diagnosis is recommended for intermittent electrical faults and PCM-related concerns.

Check for recalls

Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.

Sources

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

  2. NHTSA owner report 10745500 ↩︎ ↩︎ ↩︎ ↩︎

  3. NHTSA owner report 10745500 ↩︎ ↩︎ ↩︎

  4. NHTSA owner report 10556956 ↩︎ ↩︎

  5. NHTSA owner report 10556956 ↩︎