P2138 on 2013-2018 Chevrolet Trax: Reduced Engine Power Causes and Fixes

What's Unique About the 2013-2018 Chevrolet TRAX

For this generation of Chevrolet Trax and its platform mates (Buick Encore, Chevy Sonic), P2138 is very frequently an electrical connection problem, not a failure of the pedal sensor itself. General Motors has issued multiple Technical Service Bulletins (TSBs) about this exact issue. The bulletins point to water intrusion or poor terminal tension in a specific harness connector, X225, located in the driver's side footwell area. This water intrusion can be caused by clogged sunroof drains or leaking A-pillar seals, which allows water to drip directly onto the connector. An ALLDATA case study on a 2015 Trax also identified a corroded ground wire in connector X112 under the battery as the root cause.

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 on the instrument cluster
• Check Engine Light (Malfunction Indicator Lamp) is on.
• Poor or no acceleration.
• Vehicle may buck or jerk.
• Unresponsive or erratic throttle response.
• "Service ESC" or "Service Traction Control" message may also appear.

Most Likely Causes

1. Poor Connection at Harness Connector X225 🔴 High Probability GM has issued multiple TSBs (e.g., #07-06-04-019F) for this exact problem. The primary culprit is often water intrusion or corrosion in the instrument panel-to-body harness connector (X225), a large black connector located in the driver's kick panel area. Water leaks from sunroof drains or A-pillar seals are a common source of the moisture.
   How to confirm: Inspect the IP-to-body harness connector X225 for any signs of moisture, green or white corrosion on the pins, or backed-out pins. A 'wiggle test' on the harness while monitoring APP sensor voltages on a scan tool can often confirm an intermittent connection. The issue is often more prevalent after heavy rain.
   Typical fix: Disconnect the battery. Unplug the connector, clean the terminals thoroughly with an electrical contact cleaner, apply dielectric grease to seal out moisture, and ensure a secure connection. If corrosion is severe, the connector terminals or a section of the harness may need to be replaced.
   Est. part cost: $5-$50
2. Faulty Accelerator Pedal Position (APP) Sensor 🔴 High Probability → Shop Accelerator Pedal Assembly The APP sensor is a high-use electronic component that can wear out over time, causing its internal sensors to send conflicting signals. While common, GM TSBs specifically warn against replacing this part before checking the wiring.
   How to confirm: Use a scan tool to monitor live data for APP Sensor 1 and APP Sensor 2. As you press the pedal, the voltage readings should be smooth and change in correlation with each other. Any jumps, drops, or flat-lining indicates a faulty sensor.
   Typical fix: Replace the entire accelerator pedal assembly, which includes the integrated sensors. This is a simple repair involving one or two bolts and an electrical connector.
   Est. part cost: $70-$150
3. Damaged Wiring or Connector at the APP Sensor or Under Battery 🟡 Medium Probability The wiring near the pedal can be subject to stress or damage. A documented case on a 2015 Trax found a corroded black/violet (BLK/VIO) ground wire in connector X112 under the battery, caused by a battery acid leak.
   How to confirm: Visually inspect the wiring harness and connector going directly to the accelerator pedal for any signs of damage. Check for stable 5V reference and ground at the connector with a multimeter. For the under-battery issue, remove the battery and tray to inspect connector X112 for corrosion.
   Typical fix: Repair the damaged section of wire or replace the connector pigtail. In the case of the corroded connector under the battery, the technician bypassed the damaged ground wire.
   Est. part cost: $10-$40

Rare But Worth Checking

• Faulty Engine Control Module (ECM): → Shop Engine Control Module (ECM) This is very rare and should only be considered after all other possibilities (sensor, wiring, connectors) have been exhaustively ruled out.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P2138 is present.
2. Visually inspect the accelerator pedal assembly and its electrical connector for any obvious damage or looseness.
3. Following GM TSB #07-06-04-019F, locate and inspect the Instrument Panel (IP) to body harness connector, identified as X225. It is a large, black connector located behind the driver's side kick panel. Check for any signs of water intrusion, blue/green corrosion, or loose terminals.
4. Check for sources of water leaks, such as disconnected sunroof drain tubes or bad A-pillar seals, which are known to drip onto the X225 connector.
5. Inspect wiring under the battery for connector X112, checking for corrosion from potential battery acid leaks as noted in a 2015 Trax repair case. Specifically check the black/violet ground wire.
6. Use a scan tool to view live data for APP Sensor 1 and APP Sensor 2. Press and release the pedal slowly, watching for smooth, correlated voltage changes.
7. If the sensor data is erratic and the wiring/connectors appear clean and dry, perform a 'wiggle test' on the harness between the pedal and the firewall while watching the live data. A change in the readings indicates a wiring problem.
8. If all wiring and connectors are confirmed to be good, but the sensor data is still erratic, the Accelerator Pedal Position (APP) sensor assembly is the most likely cause of the failure.

Parts You'll Likely Need

• Accelerator Pedal Assembly (OEM #42758409) — This is the most common part replaced to fix P2138 when a wiring issue is not found. The assembly contains the two APP sensors that must correlate. The original part number may be 96892406.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $90-$150
  Aftermarket price range: $70-$120
• Dielectric Grease — Recommended by GM to protect electrical connectors from moisture and corrosion, which is a known cause of P2138 on this vehicle. Essential for the X225 connector after cleaning.
  Trusted brands: ACDelco, Permatex
  OEM price range: $5-$15
  Aftermarket price range: $5-$10

Related Codes That Often Appear With This One

• P2122
• P2123
• P2127
• P2128
• P2135

Technical Service Bulletins (TSBs) & Recalls

• 070604019F: Intermittent Malfunction Indicator Lamp (MIL) Illuminated, DTC P2138 with Reduced Engine Power (Repair Instrument Panel (IP) to Body Harness Connector).
• 07-06-04-019E: An earlier, superseded version of the TSB above, covering the same issue.

Platform-Specific Known Issues

• TSB #07-06-04-019F: IP to Body Harness Connector Corrosion: → Shop Body Wiring Harness This key bulletin from GM directly addresses P2138 with a 'Reduced Engine Power' message. It identifies the cause as a poor connection, often due to water intrusion, at the instrument panel to body harness connector (X225) and advises repairing this connector before replacing any parts.
• ALLDATA Case Study: Corroded Ground Under Battery: A documented repair on a 2015 Trax with P2138 found the cause to be a corroded black/violet ground wire in connector X112 under the battery, which had been damaged by a battery acid leak. The fix was to bypass the corroded wire.
• Owner Experience: Sunroof Drain Leak: A user on the Car Talk Community forums with a similar GM vehicle (2012 Malibu) reported the exact same symptoms (P2138, Reduced Power, Service ESC) appearing after heavy rain. The cause was a disconnected sunroof drain tube that allowed water to run down the A-pillar and onto the main harness connector, causing a short. The fix was to re-attach the drain and dry the connector.

Mechanic-Grade Diagnostic Values

• APP Sensor 1 and 2 Live Data Voltage — expected: Both sensors should show a smooth, proportional voltage change from ~0.5V (pedal released) to ~4.5V (full throttle).. Failure: A voltage divergence between the two sensor signals greater than 0.5V will typically trigger the fault. Any jumps, dropouts, or non-linear response also indicates a failure.
• 5-Volt Reference at APP Connector — expected: A steady ~5 volts at the reference wire terminal with the key on, engine off.. Failure: Voltage is significantly lower than 5V (e.g., 1-3V), which can indicate a short to ground on the 5V reference circuit, possibly caused by another sensor sharing the same circuit.
• APP Sensor Circuit Wire Resistance — expected: Less than 20 Ω on the signal and low reference wires measured from the pedal connector to the ECM connector.. Failure: Resistance is greater than 20 Ω, indicating high resistance from corrosion or an open circuit in the wiring.

Scan Tool Commands That Help

• Manual Procedure (No Tool Required): Idle Air / Throttle Relearn Procedure — After replacing the accelerator pedal, throttle body, or disconnecting the battery, the ECM may need to relearn the throttle's closed position. Failure to do this can cause continued issues even with new parts. A common manual method is to turn the ignition ON (engine off) for 3 seconds, then OFF for 10 seconds, repeat twice, then start the engine and let it idle for at least 3 minutes to learn the new values.
• GDS2 / Tech2 (or equivalent professional scanner): Idle Relearn / Throttle Relearn — This is the dealer-level method to force the ECM to relearn idle and throttle position values after component replacement. It is more reliable than the manual procedure and is the recommended method if available.

Wiring & Ground Locations

• X225 — Large, multi-wire instrument panel-to-body harness connector located behind the driver's side kick panel.. This is the primary location for corrosion and water intrusion that causes P2138, as specified in GM TSB #07-06-04-019F. The APP sensor signals pass through this connector on their way to the ECM.
• X112 — Connector located under the battery tray in the engine compartment.. A documented case study found a corroded ground wire in this connector caused P2138 on a 2015 Trax, likely due to a battery acid leak.
• GM Ground Naming Convention — Vehicle-wide. GM grounds are numbered based on location: G1xx grounds are in the engine compartment, G2xx are in the front passenger compartment/dash area, G3xx are in the rear seat area, and G4xx are in the rear cargo area. This helps in locating potentially faulty grounds affecting the ECM or sensor circuits.
• G103 — Located on the cowl in the engine compartment, typically above the brake booster.. This ground serves multiple important components, including the Body Control Module (BCM) and the Data Link Connector (DLC). A poor connection here can cause a wide range of communication and diagnostic issues.

Real Owner Repair Stories

• Tahoe Yukon Forum user (2011 Yukon XL 2500 (6.0L Engine) with 200k miles) — P2138 code with reduced engine power, APP sensor signals not correlating correctly on scan tool.
  ❌ Tried (didn't work) Replaced all six wires in the harness to the accelerator pedal., Replaced the accelerator pedal assembly., Replaced the entire throttle body assembly (including new TPS)., Verified correct 5.1V reference and 2.2Ω resistance on low reference wires.
  ✅ What actually fixed it The user's issue was unresolved after replacing all common components and verifying wiring integrity. The next diagnostic steps pointed toward an intermittent fault within a major body harness connector due to suspected water intrusion (despite no visible corrosion) or a failing Engine Control Module (ECM) as the final possibility.
• GMTNation.com forum user (2004 GMC Envoy (related GM platform)) — Reduced Engine Power light and P2138 code.
  ❌ Tried (didn't work) Replacing the accelerator pedal assembly.
  ✅ What actually fixed it The owner found water inside the C206 connector (the platform equivalent of the Trax's X225 connector) in the driver's kick panel. Cleaning the connector and letting it dry resolved the code.

When the Usual Fixes Don't Work

• While the most common fixes are cleaning connector X225 or replacing the accelerator pedal assembly, there are documented cases where these steps do not solve the problem. One owner of a similar GM vehicle replaced the pedal, the entire wiring harness to the pedal, and the throttle body, while also verifying correct voltages and resistance, yet the P2138 code persisted. This indicates the fault can sometimes be more complex, potentially stemming from an intermittent short inside a connector without visible corrosion, or, in rare cases, a failing Engine Control Module (ECM). This highlights the importance of not stopping diagnosis after replacing the most common parts if the issue remains.

OEM Part Supersession History

• 96892406 → 42758409 — GM regularly updates service part designs to integrate new materials and technologies.
  Heads up: The new part number (42758409) is the correct replacement for the original.