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…
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 P0121/P0122/P0123-type codes and erratic idle/surging.12
Most likely causes
- 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 P0121/P0122/P0123-type codes.12
- 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
- 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
- Other APPS codes stored alongside P0122 — Other APPS codes stored with P0122 (such as P0121 or P0123) point to the same pedal sensor or its wiring.3
How to diagnose
- Verify code and freeze frame; confirm P0122 is present and note pedal position % and engine conditions.
- Inspect APPS connector and wiring for corrosion, oil contamination, and chafing along the chassis harness.
- 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.
- Monitor live APPS signal while slowly pressing the pedal; note dropouts, flat spots, or low voltage that would match a low-input detection.
- Inspect grounds and power feeds to the PCM and APPS circuit for corrosion or loose terminals.
- Check for additional pedal position codes such as P0123 high input or P0121 range/performance that point to APPS-related faults.
- 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
- Digital multimeter
- Scan tool with live data
- Wiring diagram for 1998.5–2002 Ram 2500/3500 Cummins
- Contact cleaner and dielectric grease
- Basic hand tools for connector access
Tools you’ll need
- digital multimeter — Measures APPS reference and signal voltages at the connector to confirm low input and detect wiring faults.
- OBD-II scan tool — Reads P0122 and live APPS % to verify low signal and monitor sweep for dropouts.
- back-probe kit — Allows safe voltage checks at the APPS connector without damaging terminals during key-on tests.
- inspection mirror — Helps view APPS wiring harness chafing and connector corrosion along the chassis route.
Rough cost and difficulty
| Likely repair | Parts | Shop labor | Who usually does it |
|---|---|---|---|
| ECM/PCM software update | — | 0.5–1.5 hrs | Shop |
| Wiring or connector repair | $10–$100 | 1–4 hrs | DIY with tools and a manual |
| Accelerator pedal position sensor | $80–$300 | 0.3–1 hrs | Easy DIY |
| ECM/PCM replacement (programmed) | $600–$2,000 | 1–3 hrs | Shop |
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 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.
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
Repair videos: P0122 on the 5.9 Cummins 24V
Third-party videos, not ours — picked for this engine. Confirm anything against the factory service information for your truck.
Spotted an error, or something missing? Tell us
Run a 5.9 Cummins 24-Valve (VP44)? This goes straight to the editor — it is not published.