P2125 on 2007-2011 GMC Sierra Denali: Causes and Fixes for Reduced Engine Power

What's Unique About the 2007-2011 Gmc SIERRA DENALI

The GMT900 platform, which includes the 2007-2011 Sierra Denali, is known for electrical issues related to the accelerator pedal. General Motors released a Technical Service Bulletin (TSB #PIP3783D) specifically advising technicians to check the APP sensor's electrical connector for poor pin tension before replacing any parts. This means a loose connection, not a failed part, is a very common culprit for this code on these specific trucks. This issue is shared across the entire GMT900 lineup, including the Silverado, Tahoe, and Escalade.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• "Reduced Engine Power" message displayed on the instrument cluster.
• Check Engine Light (Malfunction Indicator Lamp) is illuminated.
• Poor, delayed, or non-existent throttle response.
• Vehicle may not accelerate past a certain low RPM (e.g., 1500 RPM).
• Vehicle may stall or fail to accelerate from a stop.
• Cruise control will be disabled.

Most Likely Causes

1. Poor Terminal Tension at APP Sensor Connector 🔴 High Probability This is a known issue documented by GM in TSB #PIP3783D. The connector pins can loosen from vibration, heat cycles, or being kicked in the driver's footwell, causing an intermittent signal.
   How to confirm: Disconnect the electrical connector from the accelerator pedal assembly. Visually inspect for corrosion or damage. Use a proper terminal test probe to check for a snug fit on the pins. Wiggling the harness while monitoring live sensor data on a scan tool may show the voltage dropping out. Gently tugging on each wire at the back of the connector can also reveal an internal wire break.
   Typical fix: Ensure all pins are making solid contact. If terminals are loose, they may need to be carefully re-tensioned with a small pick tool. If the connector housing is damaged or the terminals are too loose to fix, the connector pigtail must be replaced.
   Est. part cost: $10-$40 for a new connector pigtail (e.g., ACDelco PT2712, Dorman 645-595, Standard Motor Products S-1479).
2. Faulty Accelerator Pedal Position (APP) Sensor 🔴 High Probability → Shop Accelerator Pedal Assembly The sensor is an electronic component with internal potentiometers that wear out over time. Given the age of the vehicle, sensor failure is common and is the most frequent repair when wiring is confirmed to be good.
   How to confirm: Using a scan tool, monitor the live data for APP Sensor 1 and APP Sensor 2. The voltage readings should change smoothly and in correlation as you press and release the pedal. For P2125, focus on APP Sensor 2; its voltage should sweep smoothly and stay within the 0.24V to 4.49V range. Jumps, dropouts, or a frozen value on one of the sensors points to a failed pedal assembly.
   Typical fix: Replace the entire accelerator pedal assembly. The sensor is integrated into the pedal and is not sold separately. This is typically a plug-and-play part requiring no reprogramming.
   Est. part cost: $70-$190
3. Damaged Wiring Harness 🟡 Medium Probability The wiring harness runs through the driver's footwell and engine bay, making it susceptible to damage. Documented cases include wires chafing against the valve cover, rubbing on frame grounds under the driver's door, or damage from accidental kicks.
   How to confirm: Perform a detailed visual inspection of the wiring harness leading from the pedal assembly toward the firewall and in the engine bay. Look for any chafed, broken, or corroded wires. A known issue is a small black ground wire in the harness on the passenger side of the intake manifold becoming loose or broken, which can cause these symptoms. Check for continuity and shorts using a multimeter.
   Typical fix: Repair the damaged section of the wire using solder and heat shrink. In severe cases, a harness repair or replacement may be needed. Protect the repaired area with loom or tape to prevent future damage.
   Est. part cost: $5-$50 for wiring repair supplies.

Rare But Worth Checking

• Water Intrusion in Harness Connectors: GM has noted that on some vehicles, water can get into major inline harness connectors (like the body-to-dash connector), causing corrosion and multiple, difficult-to-diagnose electronic throttle control DTCs. If the pedal and its direct connector are good, these intermediate connectors should be inspected, as noted in TSB #07-06-04-019F.
• Faulty Powertrain Control Module (PCM): → Shop Engine Control Module (ECM) This is the least likely cause. The PCM should only be suspected after all other possibilities, including wiring, connectors, and the sensor itself, have been exhaustively tested and ruled out.

Diagnosis Steps

1. Scan for all DTCs to see if other related codes are present, especially P2138.
2. As per TSB #PIP3783D, the first step is to inspect the electrical connector at the accelerator pedal. Disconnect it, check for corrosion, and ensure the female terminals have good tension on the male pins.
3. With the key on and engine off, use a scan tool to monitor live data for both APP Sensor 1 and APP Sensor 2.
4. Slowly press and release the accelerator pedal. The voltage readings for both sensors should sweep smoothly without any glitches or dropouts. Specifically for P2125, ensure APP Sensor 2 voltage stays within the valid range (approx. 0.24V - 4.49V).
5. While monitoring the scan tool, wiggle the connector and harness leading to the pedal. If the voltage becomes erratic, the fault is in the wiring or connector.
6. If the wiring and connector are confirmed to be good but one of the sensor signals is still faulty, the accelerator pedal assembly has failed and needs to be replaced.
7. After replacement, clear the codes and perform a test drive. An idle learn procedure is generally not required for this part but can be performed by disconnecting the battery for 20 minutes if idling issues occur.

Parts You'll Likely Need

• Accelerator Pedal Position Sensor Assembly (OEM #25832864) — This is the most common fix for P2125 when the wiring is confirmed to be good. The sensor is integrated into the pedal assembly. This part number supersedes older numbers 15847349 and 15101848.
  Trusted brands: ACDelco, Standard Motor Products, NGK/NTK, Wells
  OEM price range: $110-$190
  Aftermarket price range: $60-$120
• Accelerator Pedal Connector Pigtail (OEM #ACDelco PT2712) — Needed if the original connector at the pedal is damaged, corroded, or the terminals have lost tension and cannot be re-pinned. Aftermarket versions include Dorman 645-595 and Standard Motor Products S-1479.
  Trusted brands: ACDelco, Dorman, Standard Motor Products
  OEM price range: $20-$40
  Aftermarket price range: $10-$25

Related Codes That Often Appear With This One

• P2138 — This code indicates a voltage correlation problem between the two APP sensors. It is very common to see with P2125 as it means the PCM is getting conflicting signals from the two sensors in the same pedal assembly.
• P2120 — Refers to a malfunction in the 'D' circuit of the APP sensor, the companion sensor to the 'E' circuit.
• P2122 / P2123 — Indicate low or high voltage on the APP Sensor 'D' circuit, respectively.
• P2127 / P2128 — Indicate low or high voltage on the APP Sensor 'E' circuit, directly related to P2125.

Technical Service Bulletins (TSBs) & Recalls

• PIP3783D: Advises inspection of the Accelerator Pedal Position (APP) circuits for poor terminal tension when codes P2120, P2122, P2123, P2125, P2127, P2128, and/or P2138 are present with a Service Engine Soon light or reduced power message.

Platform-Specific Known Issues

• A known issue documented in TSB #PIP3783D points to poor terminal tension in the accelerator pedal position sensor connector as a primary cause for P2125 and related codes, often leading to a "Reduced Engine Power" message.
• Owners have reported that even after replacing the pedal, the issue can persist, with the ultimate fix being a corroded ground wire under the driver's side door or a chafed wire in the main engine harness. A specific ground wire on the passenger side of the intake manifold has also been identified as a potential failure point.

Mechanic-Grade Diagnostic Values

• 5-Volt Reference at APP Connector (X115) — expected: 4.8 - 5.2 Volts with key on, engine off.. Failure: Voltage below 4.8V or above 5.2V points to a fault in the ECM or the reference circuit wiring.
• APP Sensor 2 Signal Voltage (Pin C) — expected: Sweeps smoothly from approx. 1.2V - 2.0V (pedal released) to approx. 2.6V - 4.5V (pedal fully depressed).. Failure: Voltage is stuck, jumps erratically, or goes below 0.24V or above 4.49V for more than 0.5 seconds.
• APP Sensor 1 Signal Voltage (Pin F) — expected: Sweeps smoothly from approx. 0.5V - 1.1V (pedal released) to approx. 2.6V - 4.5V (pedal fully depressed).. Failure: While not the direct cause of P2125, an erratic signal here will often set a companion code like P2120 or P2138.

Scan Tool Commands That Help

• Tech2 / GDS2 or equivalent professional scan tool: ETC Relearn / Throttle Relearn — After replacing the accelerator pedal assembly or throttle body, this function may be needed to reset learned values and ensure proper idle and throttle response. While often not strictly required for a pedal swap, performing it can prevent idling issues.

Wiring & Ground Locations

• X115 (APP Sensor Connector) — The 6-pin electrical connector located directly on the accelerator pedal assembly inside the cabin.. This is the primary connection point for the sensor. TSB #PIP3783D specifically calls out poor terminal tension in this connector as the main cause of P2125. All voltage and signal tests begin here. Pin C is the signal wire for APP Sensor 2, which triggers P2125.
• G103 — On the front of the passenger side cylinder head.. This is a major engine and control module ground point. A poor connection here can create a floating ground reference for the PCM, causing it to misinterpret the voltage signals from the APP sensor and set false codes.
• G104 — On the left (driver's side) rear corner of the engine, mounted to the cylinder head. A ground strap often runs from here to the firewall.. This ground is critical for the Vehicle Control Module (VCM/PCM) and various sensors. A loose or corroded G104 can cause intermittent electrical issues, including erratic sensor readings that lead to P2125 and Reduced Engine Power mode.
• G300 — Under the vehicle at the driver's door, typically on a body mount.. This is a key body and frame ground. Corrosion at this location, especially in salt-belt regions, is a known cause of various electrical faults. Forum users often trace persistent, unresolved electronic issues back to cleaning or repairing this ground connection.

Real Owner Repair Stories

• Compilation of user reports from GM-Trucks.com and SilveradoSierra.com (2008 GMC Sierra 1500, 5.3L V8) — Intermittent 'Reduced Engine Power' and 'Service Stabilitrak' messages, with codes P2125 and P2138. The issue was worse when hitting bumps or on hot days.
  ❌ Tried (didn't work) Replaced the accelerator pedal assembly with an aftermarket part., Cleaned the throttle body., Checked the main battery ground G103 on the passenger cylinder head.
  ✅ What actually fixed it The owner traced the wiring harness from the firewall to the engine and found a section of the loom had been rubbing against a metal bracket near the driver's side valve cover. Several wires, including one for the APP sensor, had chafed insulation and were intermittently shorting to ground. Repairing the damaged wires with solder and heat shrink and re-routing the harness with a zip tie permanently resolved the codes.

"I Checked Everything" — The Actual Cause

• A common failure pattern for P2125 is when a technician performs a continuity test on the APP sensor harness and finds no open circuits. However, the actual fault is a wire with chafed insulation that only shorts to ground when the engine vibrates or the harness is moved. This is why a 'wiggle test'—shaking the harness while monitoring live sensor voltage on a scan tool—is more effective than a static continuity test for finding this issue.

OEM Part Supersession History

• 15101848, 15847349 → 25832864 — Standard part evolution, likely for improved durability or material changes.
  Heads up: The replacement part 25832864 is the correct service part for the entire 2007-2011 range. However, be aware of the 'Tin Connector' variation on 2011 models.

Model Year Variations Within This Range

• 2011: Some parts listings for the accelerator pedal assembly (25832864) specifically mention 'With Tin Connector' for the 2011 model year. While the part number is the same, this suggests a possible change in the connector material late in the generation. When ordering, it is wise to visually confirm your existing connector type or ensure the replacement part specifies compatibility with the 2011 model.