Diesel Trouble Codes

P0122 — Throttle/Pedal Position Sensor/Switch A Circuit Low Input (5.9 Cummins 24-Valve (VP44))

Diagnostic reference for 5.9L Cummins 24-Valve VP44 (1998.5–2002) P0122. Focus on APPS circuit low voltage detection and related drivability symptoms. Do not…

Updated September 2, 2026

What it means

The engine control module has detected a voltage signal from the throttle or pedal position sensor that is lower than the expected range. On this platform the pedal/throttle input is an APPS (accelerator pedal position sensor), cable to a pot on the pump, and APPS failure is one of the most common faults and drives P0122/P0123/P0121/P0221-type codes and erratic idle/surging.12

Most likely causes

  1. APPS sensor internal wear / low output voltage — APPS is the pedal/throttle position input on VP44 trucks; internal wear or a failing potentiometer can pull the signal low, matching the generic low-input definition for P0122 and the platform note that APPS failure drives P0122/P0123/P0121/P0221-type codes.12
  2. APPS wiring harness chafing, corrosion or poor ground causing low voltage — Chafed or corroded wiring to the APPS can create a low-voltage condition at the PCM, consistent with the generic low-input definition. APPS faults are common on this platform and produce P0122-type codes.12
  3. PCM input circuit fault / reference voltage issue — A low reference or pull-down in the PCM input circuit can be read as a low sensor signal per the generic definition of P0122.1
  4. Related pedal position sensor B performance issue — P0221 indicates sensor/switch B range/performance problems on this platform; a related B-circuit issue can contribute to overall pedal position diagnostics alongside P0122.3

How to diagnose

  1. Verify code and freeze frame; confirm P0122 is present and note pedal position % and engine conditions.
  2. Inspect APPS connector and wiring for corrosion, oil contamination, and chafing along the chassis harness.
  3. Check APPS reference voltage and signal voltage at the connector with key on, engine off; compare to expected range and look for low signal at idle and sweep.
  4. Monitor live APPS signal while slowly pressing the pedal; note dropouts, flat spots, or low voltage that would match a low-input detection.
  5. Inspect grounds and power feeds to the PCM and APPS circuit for corrosion or loose terminals.
  6. Check for additional pedal position codes such as P0123 high input or P0221 sensor B range/performance that point to APPS-related faults.
  7. If electrical checks are inconclusive, evaluate APPS mechanical integrity and pedal linkage for binding or misalignment that could affect sensor output.

Parts and tools you may need

Rough cost and difficulty

Parts and labor vary by region and vehicle condition. APPS replacement and wiring repairs are common maintenance items on this platform; obtain quotes from qualified diesel technicians.

When to see a pro

If the APPS signal is erratic or the wiring inspection reveals harness damage near safety-critical circuits, have a qualified diesel technician inspect and repair the harness. Do not attempt repairs on high-pressure fuel or brake systems. If drivability is unsafe, stop driving and seek professional service.

Sources

  1. Curated DTC seed — 5.9 Cummins 24-Valve (VP44) (curated) ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — 5.9 Cummins 24-Valve (VP44) (curated) ↩︎ ↩︎ ↩︎

  3. Curated DTC seed — 5.9 Cummins 24-Valve (VP44) (curated) ↩︎