P2135 on 2010-2016 Buick LaCrosse: Reduced Engine Power Causes and Fixes

What's Unique About the 2010-2016 Buick LaCrosse

This issue is a well-documented weak point on many GM vehicles from this era, including the Buick LaCrosse. The failure is typically electronic, within the throttle position sensor, not due to carbon buildup. General Motors acknowledged this widespread problem by issuing Special Coverage Adjustment #14582A for numerous models, which extended the warranty for this specific failure to 10 years or 120,000 miles. While these coverages have expired for the 2010-2016 model years, they confirm a known manufacturing or design flaw with the throttle body assembly.

Symptoms You May Notice

• "Reduced Engine Power" message on the dashboard
• Check Engine Light is on
• "Service Stabilitrak" light may also illuminate
• Severely limited acceleration and speed (limp mode)
• Hesitation or surging when pressing the accelerator
• Engine may stall after stopping
• Rough or erratic idle

Most Likely Causes

1. Failed Throttle Body Assembly 🔴 High Probability → Shop Fuel Injection Throttle Body This is a known widespread issue on many GM platforms of this era, confirmed by multiple TSBs and special coverage adjustments from the manufacturer. The internal electronic throttle position sensors are integrated into the assembly, are not serviceable separately, and are prone to failure.
   How to confirm: Use a scan tool to monitor the live data for TP Sensor 1 and TP Sensor 2. The voltage readings should be smooth and correlate (one rises as the other falls) as the accelerator is pressed. If one sensor shows erratic, jumpy, or no voltage, the throttle body has failed.
   Typical fix: Replace the entire throttle body assembly with a new gasket. After replacement, a throttle relearn procedure must be performed. A DIY video guide for this specific replacement is available from 1A Auto. 🎬 Watch this step-by-step throttle body replacement guide
   Est. part cost: $150-$400
2. Faulty Accelerator Pedal Position (APP) Sensor ⚪ Low Probability → Shop Accelerator Pedal Assembly
   How to confirm: Using a scan tool, monitor the live data for the APP sensors (there are also two of them in the pedal assembly). As you press the pedal, the voltage should increase smoothly. Any dropouts or erratic readings indicate a faulty pedal sensor. One owner reported that replacing the entire pedal assembly was the ultimate fix after other attempts failed.
   Typical fix: Replace the accelerator pedal assembly.
   Est. part cost: $50-$150
3. Wiring or Connector Issues ⚪ Low Probability A GM technical information bulletin (PIP3089B) noted that for P2135, internal wire breaks can occur within 1-4 inches of the throttle body connector, even if the outer insulation looks fine.
   How to confirm: Visually inspect the wiring harness and connector at the throttle body for any signs of damage, corrosion, or loose pins. Perform a 'wiggle test' on the harness near the connector while monitoring live sensor data on a scan tool to see if the fault can be triggered.
   Typical fix: Repair the damaged wiring or clean/replace the connector pigtail.
   Est. part cost: $20-$50

Rare But Worth Checking

• Powertrain Control Module (PCM) Fault: → Shop Engine Control Module (ECM) This is very rare. Before condemning the PCM, all other possibilities, especially the throttle body and wiring, must be exhaustively ruled out.

Diagnosis Steps

1. Connect an OBD-II scanner and confirm that P2135 is the primary code.
2. Using the scanner's live data function, monitor the voltage readings of TP Sensor 1 and TP Sensor 2 while the key is on and the engine is off.
3. Slowly press and release the accelerator pedal. Observe the sensor readings. They should move smoothly and in opposite directions (e.g., one from ~4.5V down to ~0.5V, the other from ~0.5V up to ~4.5V).
4. If either sensor's reading is erratic, jumps, or is stuck, the throttle body assembly is faulty and needs to be replaced.
5. If the TP sensor readings are stable, check the Accelerator Pedal Position (APP) sensor readings in the same manner. If they are erratic, the pedal assembly is likely the cause.
6. If both sensor sets appear to function correctly on the scan tool, perform a thorough visual inspection of the wiring harness and connector going to the throttle body. Look for chafed wires, corrosion in the connector, or loose pins.
7. Perform a 'wiggle test' on the harness near the throttle body connector while watching the live data for any voltage dropouts, which would indicate an internal wire break.
8. If no issues are found, the most probable cause remains an intermittent failure of the throttle body, which is extremely common for this code.

Parts You'll Likely Need

• Throttle Body Assembly (OEM #12670981) — This is the most common failure point for code P2135 on this vehicle. The internal, non-serviceable throttle position sensors fail, requiring the replacement of the entire unit. Part number 12670981 is a common GM/ACDelco replacement for LaCrosse models with the 3.6L V6 engine.
  Trusted brands: ACDelco, Bosch, Hitachi, Dorman (OE Fix)
  OEM price range: $250-$400
  Aftermarket price range: $150-$250

Related Codes That Often Appear With This One

• P1516 — This is another GM-specific code related to Throttle Actuator Control (TAC) module performance, often triggered by the same throttle body failure.
• P2101 — This code relates to the control motor circuit range/performance within the throttle body and can appear alongside P2135.
• C0242 — This code indicates an issue with the Traction Control System. The engine computer will often disable traction/stability control when a major powertrain fault like P2135 occurs, as it cannot accurately control engine torque.

Technical Service Bulletins (TSBs) & Recalls

• TSB #11-06-04-007C: While for other GM models, this TSB details the P2135 issue and at one point recommended a sensor-only repair kit, which is less common now than full throttle body replacement.
• TSB #12-06-04-003A: Addresses DTC P2135 on various 2009-2011 GM models, indicating a correlation fault between the two TP sensors.
• TSB Bulletin #12-06-04-003: Although issued for the Buick Lucerne, this manufacturer bulletin provides a procedure for replacing the throttle position sensor cover to address a malfunction indicator lamp, reduced power message, and DTC P2135.
• Special Coverage Adjustment #14582A: This is the most significant document, confirming the widespread nature of the defect and extending warranty coverage. It specifies that Bosch-supplied throttle bodies should be replaced as a complete assembly.

Platform-Specific Known Issues

• GM issued Special Coverage Adjustment #14582A and other similar bulletins for this exact issue on many vehicles, including some LaCrosse models, extending the warranty to 10 years or 120,000 miles. This confirms it is a known factory defect, although the coverage is likely expired for most vehicles now.

Mechanic-Grade Diagnostic Values

• 5-Volt Reference Circuit Voltage at Throttle Body Connector — expected: 4.8-5.2 V. Failure: Voltage outside this range indicates an issue with the ECM or wiring.
• Low Reference Circuit Resistance to Ground at Throttle Body Connector — expected: Less than 5 ohms. Failure: Resistance greater than 5 ohms indicates an open or high resistance in the ground circuit.
• TP Sensor 1 and TP Sensor 2 Voltage Correlation — expected: The sum of TP Sensor 1 voltage and TP Sensor 2 voltage should remain relatively constant (typically around 5.0V) as the throttle is opened and closed.. Failure: The ECM will set P2135 if the two sensor signals disagree by more than 7-10% for less than a second. On a graph, the two signals should form a near-perfect 'X' pattern as the throttle is swept. Any glitches or dropouts indicate a failing sensor.
🎬 Watch how to diagnose TPS correlation issues

Scan Tool Commands That Help

• GDS2 / Tech2: Idle Learn Reset — This function must be used after replacing the throttle body assembly to reset the learned idle airflow values in the ECM, preventing high or erratic idle.
• Professional Scan Tool (GDS2, Autel, etc.): Throttle Sweep Test — This bidirectional command electronically sweeps the throttle plate while graphing the TP sensor voltages. It is used to confirm sensor functionality and can reveal intermittent glitches that are difficult to replicate by manually pressing the accelerator pedal.

Wiring & Ground Locations

• Throttle Body Connector — The main 6-pin or 8-pin electrical connector plugged directly into the electronic throttle body assembly.. This is the single point of connection for both TP sensor signals, the 5V reference, and the ground circuit. Corrosion, backed-out pins, or chafed wires within inches of this connector are a known cause of intermittent P2135 faults.
• Rear Cylinder Head Grounds — Located on the back of the passenger side cylinder head.. These grounds can be disturbed or left loose during other engine repairs. A poor ground connection here can cause erratic sensor readings and has been identified as a root cause for P2135 after other common parts were replaced.

Real Owner Repair Stories

• Tahoe Yukon Forum user (2008 GMC Yukon Hybrid (similar GMT900 electronics)) — Recurring P2135 code, especially in cold or damp weather.
  ❌ Tried (didn't work) Multiple unspecified part replacements
  ✅ What actually fixed it Found the wiring harness had rubbed through against a bracket near the ECM. Repairing the chafed wires permanently resolved the code.
• Axle's Garage YouTube Video (2010 Chevy Silverado 1500 4.8L (shares similar electronics)) — Sporadic P2135, Check Engine Light, Stability Track error, and Reduced Power mode.
  ❌ Tried (didn't work) Clearing the code with a scanner, which provided only a temporary fix.
  ✅ What actually fixed it The problem was permanently fixed by replacing only the throttle position sensor cover kit (GM P/N 19259452) on the original throttle body, as recommended by TSB #11-06-04-007C. The mechanic noted this is the correct fix in the vast majority of cases.

OEM Part Supersession History

• 12632172, 12648415, 12669016, 12671015 → 12670981 — Standard part evolution and design updates by the manufacturer.

Model Year Variations Within This Range

• 2012-2016: Models from 2012 onwards primarily used the 3.6L LFX V6 engine, which succeeded the 3.0L LF1 and 3.6L LLT engines used in 2010-2011. While the P2135 issue is common across these engines, it's crucial to verify the correct throttle body part number for the specific engine, as they may differ.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Premature Timing Chain Stretch (3.6L V6) 🔴 High — Very common, especially on earlier models. Often occurs between 70,000 and 120,000 miles. Exacerbated by extended oil change intervals. (Ref: TSB #12-06-01-009D)
• Transmission Control Module (TCM) Failure / Shifting Issues 🟠 Medium — Commonly reported issue leading to harsh shifting, slipping, or getting stuck in gear.
• Front Strut / Mount Noise 🟡 Low — A squawk or creak noise from the front suspension, especially in colder weather, is a common complaint. (Ref: TSB #09-03-08-004E)
• Water Pump/Balance Shaft Chain Rattle (2.4L I4) 🟠 Medium — On 2.4L models, a rattle or knock from the front of the engine at idle when warm is often the water pump/balance shaft chain and tensioner. (Ref: TSB #PI0202D)
• Door Lock Actuator Failure 🟡 Low — A widely reported nuisance issue where door locks fail to operate with the remote or interior switch.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: Given that the failure mode of the throttle body is electronic and related to wear on the internal sensor tracks, buying a used part is generally not recommended. The risk of the used part failing soon after installation is high, as it likely has similar wear to the part being replaced. A new OEM or high-quality aftermarket part is the most reliable long-term solution.

Donor-vehicle mileage cap: roughly under 40000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• Verify the part number matches the required OEM number for your vehicle's specific engine.
• Inspect the electrical connector pins for any signs of corrosion, damage, or pushed-out pins.
• Check the throttle plate for smooth, unrestricted movement by hand.
• If possible, source from a vehicle with the lowest possible mileage and a known history.

Aftermarket brands forum-validated for this vehicle:

• ACDelco (OEM)
• Bosch
• Hitachi
• Dorman

Brands owners have reported issues with on this vehicle:

• Unbranded, no-name parts from online marketplaces should be avoided due to inconsistent quality control and a higher likelihood of premature failure.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• C0561 — Often associated with the 'Service Stabilitrak' light that illuminates alongside P2135 when the vehicle enters limp mode.
• P0008 — Related to the premature timing chain stretch issue (TSB #12-06-01-009D) common on the 3.6L V6 engine used in the LaCrosse.
• P0017 — Another code frequently seen with the 3.6L V6 timing chain stretch, which is a high-severity known issue for this vehicle.

Frequently Asked Questions