Diesel Trouble Codes

P0123 — Throttle/Pedal Position Sensor/Switch A Circuit High Input (5.9 Cummins 24-Valve (VP44))

P0123 on the 5.9L Cummins 24-Valve VP44 means the ECM is seeing an APPS voltage above the expected range. On this platform the accelerator pedal position…

Updated September 2, 2026

What it means

The ECM has detected a voltage signal from the throttle or accelerator pedal position sensor that is higher than the expected range. On the 5.9L Cummins 24-Valve VP44 the pedal/throttle is an APPS (accelerator pedal position sensor), cable to a pot on the pump, and APPS failure is one of the most common faults on this truck and drives P0122/P0123/P0121/P0221-type codes and erratic idle/surging.12

Most likely causes

  1. APPS sensor internal failure / worn resistive track — High voltage from the APPS is the classic presentation for P0123 on this platform. APPS failure is one of the most common faults and drives P0122/P0123/P0121/P0221-type codes.2
  2. APPS wiring harness chafing, corrosion, or poor ground causing voltage pull-up — Chafed or corroded APPS wiring can raise the signal voltage seen by the ECM. The generic definition for P0123 is ECM detecting a voltage signal from the throttle/pedal position sensor higher than expected.1
  3. ECM/PCM internal reference voltage or processing fault — A high reference voltage or internal ECM fault can make a good APPS appear high. Control module processor performance issues are documented for this platform.2
  4. Incorrect APPS adjustment or mounting after pedal/pump work — Misadjusted APPS can sit outside the calibrated voltage window, triggering high-input detection. APPS failure commonly drives P0122/P0123/P0121/P0221-type codes.2

How to diagnose

  1. Verify code and freeze frame; confirm P0123 is present and note pedal position and engine conditions at set.
  2. Inspect APPS connector and wiring for corrosion, oil contamination, chafing, and secure routing; repair as needed.
  3. Check APPS reference voltage and signal voltage at the sensor connector with a scan tool and multimeter; compare to spec and monitor sweep with pedal movement.
  4. Perform a wiggle test on the APPS harness while watching live data for voltage spikes or dropouts.
  5. Inspect ECM connectors and grounds for corrosion/loose pins; clean and re-seat as needed.
  6. If voltages are normal at the sensor but high at the ECM, suspect harness short to voltage or ECM internal fault.
  7. Clear codes, road test with live data logging of APPS voltage to confirm the fault is resolved.
  8. Note: Do not attempt step-by-step repair of safety-critical systems. Focus on diagnosis and refer to service information for component replacement procedures.

Parts and tools you may need

Rough cost and difficulty

APPS replacement is a common repair on this platform; parts and labor vary by region and shop. NHTSA complaints document high repair costs for VP44-related failures and other issues on these trucks.

When to see a pro

If APPS voltage remains high after harness inspection, or if ECM reference voltage is out of spec, have a qualified diesel technician evaluate the ECM and APPS circuit with factory diagnostics. Do not attempt high-pressure fuel or ECM programming procedures without proper training and tools.

Sources

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

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