P2123 on 2007-2014 Chevrolet Tahoe: Causes and Fixes for 'Reduced Engine Power'

What's Unique About the 2007-2014 Chevrolet TAHOE

For this generation of Tahoe and its GM truck siblings (GMT900 platform), the cause of P2123 is well-documented. GM issued Technical Service Bulletin #PIP3783D specifically identifying 'poor terminal tension' in the accelerator pedal's electrical connector as a common culprit. This means that before you rush to replace the pedal itself, a simple wiggle test or cleaning of the connector might solve the problem. Many owners experience the dreaded 'Reduced Engine Power' message as a direct result of this known wiring sensitivity.

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 is illuminated.
• Poor, delayed, or no throttle response.
• Vehicle will not accelerate past a certain low speed or RPM.
• "Service StabiliTrak" and "Service Traction Control" messages may also appear. This happens because the ECU cannot trust the driver's torque request from the faulty pedal sensor, so it disables the stability and traction systems as a precaution.

Most Likely Causes

1. Faulty Accelerator Pedal Position (APP) Sensor Assembly 🔴 High Probability The sensor is a high-use electronic component. Its internal contacts wear out over time, leading to erratic voltage signals that can spike high, triggering the P2123 code. It is a sealed, non-serviceable unit.
   How to confirm: After confirming wiring is intact, use a scan tool to monitor APP Sensor 1 voltage. 🎬 Watch: How to test your pedal and see a glitched signal. If it's stuck high (over 4.75V) or shows erratic spikes/glitches when the pedal is pressed and released, the sensor is bad.
   Typical fix: Replace the entire accelerator pedal assembly. It is a single unit held by two 10mm bolts.
   Est. part cost: $90-$125
2. Poor Wiring Connection at the APP Sensor 🟡 Medium Probability As documented in TSB #PIP3783D, the female terminals in the 6-pin connector at the pedal can lose tension, causing an intermittent or poor connection. This is a very common failure point for the entire GMT900 platform.
   How to confirm: Visually inspect the connector for corrosion or backed-out pins. With the key on, gently wiggle the connector and harness while observing live data on a scan tool to see if the voltage reading for APP Sensor 1 changes erratically.
   Typical fix: Clean the connector terminals with electrical contact cleaner and apply dielectric grease. If terminals are loose, they may need to be carefully re-tensioned with a pick tool, or the connector pigtail (e.g., Standard Motor Products S-1479, ACDelco PT2712) may need to be replaced.
   Est. part cost: $0-$40
3. Damaged Wiring Harness ⚪ Low Probability Wires can get damaged from abrasion, being pinched, or corrosion. TSB #PIP3783D specifically calls out checking connectors C109/X109 (under the fuse block) and C205/X205 (under the instrument panel trim) for issues. Owners have also found broken ground wires near the firewall.
   How to confirm: Perform a continuity and resistance check on the signal (Dark Blue wire), reference, and ground (Brown wire) circuits between the APP sensor connector and the ECM connector.
   Typical fix: Repair the specific broken or shorted wire in the harness. This may involve soldering and heat-shrinking the damaged section.
   Est. part cost: $5-$50

Rare But Worth Checking

• Faulty Powertrain Control Module (PCM): This is very rare. Before condemning the PCM, all other possibilities, especially wiring and the sensor itself, must be exhaustively ruled out. A PCM failure often presents with multiple, unrelated codes.
• Poor Ground Connection: A corroded or loose main ground wire on the frame under the driver's side door can cause fluctuating voltages across multiple systems, including the APP sensor. One owner on a forum fixed their recurring P2123 code by cleaning this ground connection after a new pedal failed to solve the issue.

Diagnosis Steps

1. Read all stored DTCs with an OBD-II scanner. Note all codes present, paying special attention to P2138, P2122, or P2127/P2128.
2. Using the scanner's live data function, monitor the voltage readings for both APP Sensor 1 and APP Sensor 2.
3. With the key on and engine off, slowly press and release the accelerator pedal. APP Sensor 1 voltage should sweep smoothly (e.g., ~0.9V to ~2.4V) and APP Sensor 2 should also sweep smoothly (e.g., ~3.1V to ~2.6V). The key is a smooth change with no spikes or dropouts.
4. For P2123, you are looking for the APP Sensor 1 ('D' circuit) voltage to be stuck high (e.g., >4.75V) or to have sharp spikes.
5. If the voltage is abnormal, locate the accelerator pedal assembly and visually inspect its 6-pin electrical connector for loose pins, corrosion, or damage.
6. Gently wiggle the connector and harness at the pedal while watching the live data to see if the voltage reading stabilizes or jumps around, indicating a connection issue.
7. If the pedal connector seems fine, follow TSB #PIP3783D and inspect harness connectors C109/X109 (below the underhood fuse block) and C205/X205 (under the dash) for similar issues.
8. If the connector and harness wiggle tests point to a connection issue, clean the terminals or replace the pigtail. If no issue is found, use a multimeter to verify a stable 5-volt reference and a good ground at the pedal connector.
9. If all wiring, connectors, power, and ground are confirmed to be good, the accelerator pedal assembly itself is faulty and should be replaced.

Parts You'll Likely Need

• Accelerator Pedal with Position Sensor (OEM #25832864) — This is the most common failed component for code P2123. The sensor is integrated into the pedal assembly and is not sold separately. This part number also supersedes older part numbers 15847349 and 15101848.
  Trusted brands: ACDelco / GM Genuine Parts, Dorman, Standard Motor Products (SMP)
  OEM price range: $95-$125
  Aftermarket price range: $70-$100
• APP Sensor Connector Pigtail (OEM #ACDelco PT2712 / Standard Motor Products S-1479) — Needed if the original connector is damaged or the terminals have lost tension and cannot be reliably repaired.
  Trusted brands: ACDelco, Dorman, Standard Motor Products (SMP)
  OEM price range: $25-$40
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P2138 — This code indicates a correlation error between APP Sensor 1 and APP Sensor 2. It's very common to see with P2123 as a high voltage reading on one sensor will not match the expected reading of the other, signaling a conflict to the ECM.
• P2122 — This is the opposite code, for 'APP Sensor 'D' Circuit Low Input'. Seeing both can indicate a very erratic connection or a failing sensor causing voltage to swing from one extreme to the other.
• P2127 / P2128 — These are the low and high input codes for the second sensor (APP Sensor 2, or circuit 'E'). Seeing these along with P2123 can point to a problem with shared wiring, like the 5-volt reference or ground circuit, or a complete failure of the pedal assembly.

Technical Service Bulletins (TSBs) & Recalls

• PIP3783D: Mentions that for codes including P2123, P2122, P2127, P2128, and P2138 accompanied by a 'Reduced Engine Power' message, technicians should inspect the Accelerator Pedal Position circuits for poor terminal tension at the pedal, ECM, and intermediate harness connectors.

Platform-Specific Known Issues

• A known issue, documented in TSB #PIP3783D, involves poor terminal tension in the APP sensor electrical connector, leading to intermittent signals and triggering codes like P2123. The bulletin advises checking terminals at the pedal, ECM, and intermediate connectors C109/X109 and C205/X205.
• Water intrusion in the driver's side kick panel or from a leaky roof antenna can corrode main body harness connectors (like C205/X205), which can also cause this code and related symptoms.

Mechanic-Grade Diagnostic Values

• APP Sensor 1 Signal Voltage (Circuit 'D') — expected: Approx. 0.6V - 0.9V with pedal released, sweeping smoothly to 3.9V - 4.7V when fully depressed.. Failure: Voltage is stuck high, typically >4.75V, or does not change smoothly with pedal movement.
• APP Sensor 2 Signal Voltage (Circuit 'E') — expected: Approx. 0.3V - 0.6V with pedal released, sweeping smoothly to 1.95V - 2.4V when fully depressed.. Failure: Voltage does not correlate with Sensor 1, or shows jumps, dropouts, or flat spots.
• 5-Volt Reference at APP Connector — expected: 4.8 - 5.2 Volts with Key On, Engine Off.. Failure: Voltage is less than 4.8V or greater than 5.2V, indicating a circuit or PCM issue.
• Low Reference (Ground) Circuit Resistance — expected: Less than 5 ohms between the low reference circuit terminal and a known good ground.. Failure: Resistance is greater than 5 ohms, indicating an open or high resistance in the ground circuit.

Scan Tool Commands That Help

• Tech2 / GDS2: Live Data Monitoring of APP Sensor 1 and APP Sensor 2 Voltage — This is the primary diagnostic step to visually confirm if the sensor voltage is stuck high or not sweeping smoothly when the pedal is pressed, directly identifying the fault related to P2123.

Wiring & Ground Locations

• APP Sensor Connector (6-Pin) — At the top of the accelerator pedal assembly under the dashboard.. This is the primary connection point. TSB #PIP3783D identifies poor terminal tension here as a main cause. Pin E (Dark Blue wire) is the APP Sensor 1 signal circuit that sets P2123.
• G103 — On the front of the passenger side (right) cylinder head.. This is a major engine ground. While not the direct ground for the APP sensor's low reference circuit, a poor connection here can introduce electrical noise and voltage offsets that affect multiple sensors, including the APP.
• G104 — On the left rear corner of the engine, mounted to the left cylinder head.. Another critical engine ground point that can cause widespread sensor issues if compromised.
• Ground under driver's side door — On the frame, under the driver's side door area.. A corroded or loose frame ground in this location has been reported by owners to cause various electrical issues, including APP sensor codes, after other common fixes have failed.

Real Owner Repair Stories

• YouTube user 'Rainman Ray's Repairs' (GM Truck (Silverado/Sierra/Tahoe platform)) — Reduced Engine Power message, vehicle would lose power and go into limp mode (5 mph), especially when hitting bumps.
  ❌ Tried (didn't work) Adding multiple extra ground wires from the engine to the chassis.
  ✅ What actually fixed it Found a small black ground wire coming out of the main harness on the passenger side of the intake manifold. The wire was broken internally at its connection point. Splicing and repairing this single broken ground wire resolved all symptoms.
• TahoeYukonForum.com user (GMT900 Tahoe/Yukon) — Weird electrical problems, dash lights and messages, bad fuel mileage, and recurring voltage-related codes.
  ❌ Tried (didn't work) Replacing the accelerator pedal assembly.
  ✅ What actually fixed it The user found two ground points on the frame under the driver's side door. One had fallen off completely due to rust. Cleaning the frame to bare metal and re-securing the ground connection with a new stainless steel bolt solved the problems.

When the Usual Fixes Don't Work

• While replacing the accelerator pedal assembly is the most common fix, there are numerous documented cases where this did not solve the problem. In these instances, the root cause was often traced back to the vehicle's wiring. A frequently missed failure point is a small, black ground wire located in the harness on the passenger side of the intake manifold, which can break internally and cause intermittent 'Reduced Engine Power' events, especially when hitting bumps. Another confirmed fix, after a new pedal failed to work, was cleaning and re-securing corroded main ground points on the vehicle's frame under the driver's side door. These cases highlight that a thorough inspection of all related grounds and wiring is critical before and after replacing parts.

OEM Part Supersession History

• 15101848, 15847349 → 25832864 — Part has been updated by the manufacturer over time.
  Heads up: The newest part number, 25832864, is the correct replacement for the entire 2007-2014 generation and is backward compatible with the older numbers.

Model Year Variations Within This Range

• 2009-2014: Starting in 2009, the Tahoe received a 6-speed automatic transmission (6L80) paired with the 5.3L V8, replacing the previous 4-speed. This does not directly affect the cause or diagnosis of P2123, as the accelerator pedal and its communication with the ECM remain functionally the same.