P2123 on 2007-2011 GMC Yukon Denali: Causes and Fixes for Reduced Power

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

On this generation of GM trucks and SUVs (the GMT900 platform), issues with the accelerator pedal sensor are extremely well-documented. GM issued a Technical Service Bulletin (TSB #PIP3783D) that specifically identifies "poor terminal tension" in the accelerator pedal's electrical connector as a common cause for codes like P2123. This means that while the pedal sensor itself frequently fails, the problem can sometimes be just a simple wiring connection issue at the pedal. Furthermore, another TSB (#07-06-04-019F) points to water intrusion in the driver's kick panel corroding a major harness connector, which can also cause this exact code and symptom set.

Diagnostic Flowchart

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

Symptoms You May Notice

• Check Engine Light is on
• "Reduced Engine Power" message on the dashboard
• Poor or no throttle response
• Vehicle will not accelerate properly
• StabiliTrak or traction control warning lights may illuminate simultaneously
• Unstable or high engine idle

Most Likely Causes

1. Failed Accelerator Pedal Position (APP) Sensor Assembly 🔴 High Probability → Shop Accelerator Pedal Assembly The sensor is an electronic component that wears out over time and is a very common failure item on this platform. The internal potentiometers fail, causing erratic voltage signals.
   How to confirm: Use a scan tool to monitor the voltage for APP Sensor 1 and APP Sensor 2. As you press the pedal, the voltage for APP1 will likely be stuck high (around 4.8V or higher) or respond erratically, while APP2 may read normally. A normal APP1 sensor sweeps smoothly from approximately 0.6-0.9V to 3.9-4.7V.
   Typical fix: Replace the entire accelerator pedal assembly. The sensor is integrated into the pedal and is not sold separately. The job typically requires only a 10mm socket or T30 Torx bit and takes less than 30 minutes. 🎬 Watch: Step-by-step guide to replacing the accelerator pedal assembly.
   Est. part cost: $60-$150
2. Poor Connection or Damaged Wiring at the APP Sensor 🟡 Medium Probability GM issued TSB #PIP3783D specifically for poor terminal tension in the APP sensor connector. Corrosion from water intrusion near the driver's kick panel can also cause issues in a different major harness connector, per TSB 07-06-04-019F. 🎬 Watch: How to find and stop water leaks near the floorboards. Wires can also break or fray inside the harness near the firewall.
   How to confirm: Visually inspect the 6-pin connector at the accelerator pedal. Check for loose, bent, or corroded pins. Wiggle the harness while monitoring the sensor voltage on a scan tool to see if the reading changes. A user on Chevrolet Forum found the ultimate cause to be a broken ground wire just inside the firewall. Also, disconnect and inspect the large IP-to-body harness connector in the driver's kick panel for any green or white corrosion.
   Typical fix: Clean the connector terminals and apply dielectric grease. If terminal tension is poor, the female pins in the connector can be carefully re-tensioned or the connector pigtail can be replaced. Repair any damaged or broken wires found. In one owner's case, repairing a single broken ground wire resolved the issue completely.
   Est. part cost: $0-$50
3. Water Intrusion Corroding IP-to-Body Harness Connector 🟡 Medium Probability A known leak path from the sunroof drains, windshield seal, or roof rack can allow water to drip onto the main Instrument Panel (IP) to body harness connector located in the driver's kick panel. This is documented in TSB 07-06-04-019F.
   How to confirm: Pull back the carpet and trim in the driver's side footwell, near the emergency brake pedal. Locate and disconnect the large, multi-pin electrical connectors (often black or gray). Inspect both halves of the connector thoroughly for signs of moisture, or green and white crusty corrosion on the pins.
   Typical fix: If corrosion is found, the connector must be cleaned meticulously with an electrical contact cleaner and a small brush. If pins are severely damaged, they may need to be replaced with new terminated leads. The source of the water leak must also be identified and repaired to prevent a recurrence.
   Est. part cost: $10-$75

Rare But Worth Checking

• Powertrain Control Module (PCM) Failure: This is very rare. Before suspecting the PCM, all other possibilities, especially the pedal assembly and its wiring, must be exhaustively ruled out. A PCM failure might also be accompanied by other unrelated trouble codes.

Diagnosis Steps

1. Read all diagnostic trouble codes (DTCs) with an OBD-II scanner. Note any other codes present, especially P2138.
2. Use the scanner's live data function to monitor the voltage readings of both APP Sensor 1 and APP Sensor 2.
3. Press and release the accelerator pedal slowly, observing the voltage changes. Both should move smoothly. For P2123, you will likely see APP1 stuck at a high voltage (e.g., >4.75V).
4. Turn off the vehicle. Visually inspect the electrical connector and wiring harness at the accelerator pedal for any signs of damage, corrosion, or loose pins. This is a known issue per TSB #PIP3783D. Wiggle the harness while a helper watches the live data to check for intermittent connections.
5. CRITICAL: Inspect the driver's side footwell and kick panel area for signs of moisture. Disconnect the large IP-to-body harness connectors and check for green or white corrosion as described in TSB 07-06-04-019F.
6. If the wiring and connectors (both at the pedal and in the kick panel) look good, the most likely cause is a failed APP sensor.
7. Replace the accelerator pedal assembly, which is typically held on by two 10mm nuts or T30 Torx bolts.
8. Reconnect the battery if it was disconnected, clear the codes, and perform a test drive to ensure the issue is resolved.

Parts You'll Likely Need

• Accelerator Pedal Assembly (OEM #25832864 (supersedes 15847349, 15101848)) — This is the most common cause of P2123. The sensor is integrated into the pedal assembly and cannot be replaced separately. Note that part numbers may vary for vehicles with/without power-adjustable pedals (RPO code JF4).
  Trusted brands: ACDelco, GM Genuine Parts, Dorman, Standard Motor Products, NGK/NTK
  OEM price range: $95-$160
  Aftermarket price range: $60-$100
• APP Sensor Connector Pigtail (OEM #Varies) — If the connector at the pedal is damaged or the terminals have lost tension and cannot be repaired, replacing the pigtail is the next step.
  Trusted brands: ACDelco, Dorman, Standard Motor Products
  OEM price range: $25-$40
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P2122 — This is the code for 'APP Sensor 1 Circuit Low Input'. Seeing both P2122 and P2123 can indicate an intermittent short or a failing sensor with erratic voltage output.
• P2127 / P2128 — These codes relate to the second sensor (APP Sensor 2) in the pedal assembly. If there is a problem with the shared 5-volt reference or ground wire in the harness, it can cause faults on both sensor circuits.
• P2138 — This code indicates a correlation error between APP Sensor 1 and APP Sensor 2. It is very common to see this code alongside P2123, as the high voltage from one sensor will not match the expected reading from the other, triggering the ECM's safety logic.

Technical Service Bulletins (TSBs) & Recalls

• PIP3783D: Addresses a Service Engine Soon lamp or reduced engine power message with various APP sensor codes (including P2123), advising technicians to inspect the accelerator pedal position circuits for poor terminal tension.
• 07-06-04-019F: Details issues of water intrusion in the left front door/kick panel area causing various electrical problems, including codes P2123 and P2138.

Platform-Specific Known Issues

• A known issue documented in GM Technical Service Bulletin #PIP3783D points to "poor terminal tension" in the accelerator pedal position sensor's electrical connector, which can cause this code without the pedal itself being faulty.
• A second known issue, documented in TSB #07-06-04-019F, is water intrusion in the driver's kick panel area causing corrosion on a major body harness connector, which can mimic a failed pedal sensor perfectly.
• One owner on Chevrolet Forum with code P2123 on a similar GM truck platform traced the problem to a broken ground wire for the APP sensor circuit located just inside the firewall.

Mechanic-Grade Diagnostic Values

• APP Sensor 1 Signal Voltage (D-Circuit) — expected: Approx. 0.6 - 0.9V with pedal released, sweeping smoothly to 3.9 - 4.7V when fully depressed.. Failure: Voltage is stuck high (above 4.75V) or does not change smoothly with pedal movement.
• APP Sensor 2 Signal Voltage (E-Circuit) — expected: Approx. 0.3 - 0.6V with pedal released, sweeping smoothly to 1.95 - 2.4V when fully depressed.. Failure: Voltage does not correlate with Sensor 1, or shows jumps and dropouts.
• 5-Volt Reference Circuit at APP Harness Connector — expected: 4.8 - 5.2 Volts with Key On, Engine Off.. Failure: Voltage is less than 4.8V (indicates open/short to ground) or greater than 5.2V (indicates short to voltage).
• Low Reference (Ground) Circuit Resistance at APP Harness Connector — expected: Less than 5.0 ohms between the ground pin and a known-good chassis ground.. Failure: Resistance is greater than 5.0 ohms, indicating an open or high resistance in the ground circuit.
• APP Sensor 1 Signal Voltage on Scan Tool (Harness Disconnected) — expected: Less than 0.1 Volts.. Failure: If the voltage is greater than 0.1V with the sensor disconnected, it indicates the signal wire is shorted to voltage somewhere in the harness or there is an internal ECM fault.

Scan Tool Commands That Help

• Tech2 / GDS2: Live Data Monitoring (APP Sensor 1 & 2 Voltage) — This is the primary diagnostic step to visually confirm if the APP Sensor 1 voltage is stuck high or responding erratically compared to APP Sensor 2 when the pedal is pressed.
• Tech2 / GDS2: Clear DTCs — Used after a repair is completed to clear the P2123 code and turn off the Check Engine Light. A test drive is required to confirm the fault does not return.

Wiring & Ground Locations

• APP Connector Pin E — At the 6-pin connector on the accelerator pedal assembly.. This pin carries the signal for APP Sensor 1 via a Dark Blue wire. A high voltage reading on this specific circuit is what triggers code P2123.
• G103 — On the front of the passenger side (right) cylinder head.. This is a major engine and ECM ground. While not the direct ground for the APP sensor, a poor connection here can introduce electrical noise and voltage offsets that affect multiple sensors, potentially causing incorrect readings.
• G300 — Under the vehicle on a body mount, near the driver's side door area.. A forum user reported fixing recurring APP sensor issues by finding and cleaning a corroded frame ground under the driver's side door; G300 is a likely candidate for this ground point.

Real Owner Repair Stories

• Chevrolet Forum user (2006 Trailblazer (Similar GM Platform)) — P2123 code, Stabilitrak light.
  ❌ Tried (didn't work) Initial diagnosis pointed towards the pedal assembly itself.
  ✅ What actually fixed it The owner found and repaired a broken ground wire for the APP sensor circuit located just inside the firewall.
• Tahoe Yukon Forum user (2007-2014 GM SUV) — Recurring 'Reduced Engine Power' message and APP sensor codes.
  ❌ Tried (didn't work) Replacing the accelerator pedal assembly did not solve the problem.
  ✅ What actually fixed it The owner cleaned the main ground wires attached to the frame under the driver's side door, which resolved the voltage-related codes.

When the Usual Fixes Don't Work

• While replacing the accelerator pedal assembly is the most common fix, there are multiple documented cases where this did not solve the problem. In one instance, the actual cause was a broken ground wire for the sensor circuit found inside the harness near the firewall. In another case, a new pedal assembly failed to fix the issue, but cleaning a corroded main ground on the vehicle's frame under the driver's door resolved the recurring codes. These examples highlight the importance of verifying the integrity of the sensor's electrical circuits, including its ground path, before and after replacing parts.

OEM Part Supersession History

• 15101848, 15847349 → 25832864 — Part has been updated by the manufacturer over time.
  Heads up: Part 25832864 is specified for vehicles WITH power-adjustable pedals. A different part number may be required for vehicles without this option. Always verify fitment with a VIN.