P1125 — Accelerator Pedal Position (APP) System — LB7 Duramax
Diagnostic reference for P1125 on LB7 Duramax (2001–2004).
What it means
GM-enhanced code: the ECM has detected a fault in the accelerator pedal position sensor system. On the LB7-era GM trucks the APP sensor at the pedal and its wiring are the usual cause; expect reduced power or idle-only operation.1
Most likely causes
- APP sensor internal failure or out-of-range signal — The pedal-mounted APP sensor is producing a voltage or resistance outside the ECM’s expected range, triggering the APP system fault.1
- APP sensor wiring/connector damage or corrosion — Chafed, pinched, or corroded wiring and connector terminals at the pedal or along the harness cause intermittent or open APP signals.1
- Poor ground or reference voltage supply to the APP sensor — Loss of the 5 V reference or ground to the APP sensor results in implausible pedal position readings.1
How to diagnose
- Verify the code and freeze-frame data to confirm the fault occurs under pedal movement.
- Inspect the accelerator pedal assembly and APP sensor connector for corrosion, loose terminals, or damaged wiring.
- Check APP sensor reference voltage, ground, and signal voltage sweep with a scan tool while slowly pressing the pedal; look for smooth, linear change and values within spec.
- Inspect the harness routing from the pedal to the ECM for chafing, pinching, or heat damage, especially near the firewall and under the dash.
- If voltages are erratic, disconnect the APP sensor and test sensor resistance sweep per service information; replace the pedal assembly if the sweep is non-linear or out of range.
- Clear codes and road test to confirm the APP system fault is resolved and pedal response is normal.
Parts and tools you may need
- Digital multimeter
- Scan tool with live data
- Wiring diagram for LB7 chassis
- Replacement accelerator pedal assembly with integrated APP sensor
Tools you’ll need
- OBD-II scan tool — Reads P1125 and freeze-frame data and provides live APP sensor voltage sweep to verify smooth pedal signal during diagnosis.
- digital multimeter — Checks APP sensor reference voltage, ground continuity, and signal voltage sweep against spec to identify wiring or sensor faults.
- inspection mirror — Lets you see the APP sensor connector and harness routing under the dash and near the firewall for corrosion or chafing without full disassembly.
- terminal release tool set — Safely disconnects and reseats the APP sensor connector to inspect terminals for corrosion or damage without harming the pins.
Rough cost and difficulty
| Likely repair | Parts | Shop labor | Who usually does it |
|---|---|---|---|
| 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 |
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 pedal voltages check normal but the code returns, have the ECM and harness tested by a qualified diesel technician to rule out internal ECM faults or intermittent wiring issues.
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
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