P1515 on 2002-2009 GMC Envoy 4.2L: Reduced Power Causes and Fixes

What's Unique About the 2002-2009 GMC Envoy

The GMT360 platform, which includes the Envoy and its sibling the Chevrolet Trailblazer, is known for electronic throttle body issues on the 4.2L LL8 engine. These problems are so common that most owners will experience them at some point. The issues often stem from three main areas: heavy carbon buildup that causes the throttle plate to stick, internal failure of the throttle body's electronic sensors or motor, and brittleness of the wiring harness near the throttle body connector due to years of heat and vibration. A less common but notable cause on this platform is a short in a shared 5-volt reference circuit, sometimes caused by a failing fan clutch.

Symptoms You May Notice

• "Reduced Engine Power" message on the dashboard
• Check Engine Light is on
• Extremely limited acceleration or throttle response ('limp mode')
• Unstable or rough idle
• Vehicle may stall when coming to a stop.
• Inability to accelerate past a low speed.

Most Likely Causes

1. Dirty or Sticking Throttle Body 🔴 High Probability → Shop Fuel Injection Throttle Body The 4.2L LL8 engine's crankcase ventilation system can deposit oil vapor and carbon onto the throttle body over time, causing the throttle plate to bind and not respond correctly to PCM commands. This is the most common first thing to check.
   How to confirm: Remove the air intake tube connected to the throttle body. Visually inspect the throttle plate and bore for a thick, black layer of carbon buildup. Manually (with the ignition off) push the plate to feel for any sticking or binding; it should move smoothly with spring pressure and return to a slightly open position.
   Typical fix: Remove the throttle body from the intake manifold and clean it thoroughly on both sides with dedicated throttle body cleaner and a soft brush or rag. Cleaning it while still on the vehicle is often not sufficient. Replace the gasket upon reinstallation.
   Est. part cost: $5-$15
2. Failing Throttle Body Assembly 🔴 High Probability → Shop Fuel Injection Throttle Body The integrated Throttle Position Sensors (TPS) and actuator motor inside the assembly wear out over time, leading to dead spots, erratic signals, or complete failure. This is a very common failure item after a cleaning does not resolve the issue.
   How to confirm: If a thorough cleaning doesn't fix the code, the issue is very likely the throttle body itself. A professional can use a scan tool to graph the commanded vs. actual throttle position; any jumps, dropouts, or disagreements point to a bad throttle body. Owners often report that the problem is intermittent at first.
   Typical fix: Replace the entire throttle body assembly. The sensors and motor are not serviced separately on this platform. 🎬 See this walkthrough for replacing the entire throttle body assembly. A relearn procedure is required after replacement, which can be accomplished by disconnecting the battery for at least 30 minutes to reset the PCM or by following a specific idle sequence.
   Est. part cost: $150-$400
3. Wiring Harness or Connector Issues 🟡 Medium Probability The engine's heat and vibration can make the wires near the throttle body connector brittle. The copper strands can break inside the insulation, causing an intermittent open circuit that is hard to see. This is a well-documented issue in owner forums.
   How to confirm: With the engine idling, carefully wiggle the wiring harness near the throttle body connector. If this causes the engine to stumble, the idle to change, or the code to set, you have found the problem area. Visually inspect the connector for backed-out pins, corrosion, or damage. Pay close attention to the harness where it may rub or be secured tightly.
   Typical fix: Repair the broken wire(s) or replace the connector pigtail. Soldering and heat-shrinking the repair is recommended for durability. Common pigtail part numbers include ACDelco PT2628 or Dorman 645-203.
   Est. part cost: $15-$50
4. Poor Ground Connections ⚪ Low Probability Corrosion can build up on major engine and chassis ground points over time, creating unwanted resistance and causing intermittent voltage drops to the PCM and throttle control system. Key grounds for the GMT360 platform are often cited as sources of electrical gremlins.
   How to confirm: Locate the main ground straps/wires. On the GMT360 platform, key locations include G103 (cowl, above brake booster) and G104 (rear of driver's side cylinder head). Inspect for looseness, fraying, or corrosion. Use a multimeter to check for voltage drops between the battery negative terminal and the engine block/chassis.
   Typical fix: Remove, clean all contact surfaces with a wire brush, and securely re-fasten all affected ground connections. It may be beneficial to add a redundant ground wire as a preventative measure.
   Est. part cost: $0-$10

Rare But Worth Checking

• Faulty Accelerator Pedal Position (APP) Sensor: → Shop Accelerator Pedal Assembly While this usually sets different codes (like P1221, P1271, or P2138), a fault in the pedal sensor can sometimes contribute to general throttle system errors as it shares a 5-volt reference circuit with other components. It's less likely for P1515 specifically but is part of the same system.
• Faulty Powertrain Control Module (PCM): This is very rare. The PCM should only be considered a cause after all other possibilities—throttle body, wiring, and grounds—have been exhaustively ruled out. A failing PCM can cause a wide range of seemingly unrelated codes.
• Shorted 5V Reference Circuit (e.g., Fan Clutch): A short circuit in another component that shares the 5-volt reference line with the throttle body can cause this code. On the GMT360 platform, a known, though uncommon, culprit is the electronically controlled fan clutch. Unplugging the fan clutch connector and seeing if the code disappears is a valid diagnostic step.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P1515 and any other related codes.
2. Visually inspect the throttle body for heavy carbon buildup. This is the most common cause.
3. If dirty, remove the throttle body and clean it thoroughly. Reinstall with a new gasket and perform a throttle relearn procedure by letting the engine idle for 3 minutes, turning it off for 60 seconds, and idling again for 3 minutes.
🎬 Watch: How to manually reset and relearn your GM throttle body.
4. If the code returns, inspect the wiring harness and connector at the throttle body. With the engine running, wiggle the wires to check for intermittent faults. Check for broken wires or damaged pins.
5. Check and clean major engine and chassis ground points for corrosion or looseness, specifically G103 and G104.
6. If wiring and grounds are good, consider unplugging the electronic fan clutch connector to rule out a short on the shared 5V reference circuit. Clear codes and see if they return.
7. If cleaning, wiring, and grounds are all confirmed good, the throttle body assembly itself is highly suspect and should be replaced.
8. As a final, less common step, diagnose the Accelerator Pedal Position (APP) sensor and the PCM.

Parts You'll Likely Need

• Throttle Body Assembly (OEM #12694872) — This is the most common fix when a thorough cleaning does not resolve the P1515 code. The internal sensors and motor fail over time. The OEM part number 12694872 is a genuine GM part.
  Trusted brands: ACDelco (OEM), Hitachi, Standard Motor Products (SMP), Cardone (Remanufactured)
  OEM price range: $250-$400
  Aftermarket price range: $150-$250
• Throttle Body Gasket (OEM #8890561) — Must be replaced whenever the throttle body is removed to prevent vacuum leaks.
  Trusted brands: ACDelco, Fel-Pro, Mahle
  OEM price range: $10-$20
  Aftermarket price range: $5-$15
• Throttle Body Connector Pigtail (OEM #88986221) — The wiring near the connector becomes brittle from heat and vibration, causing intermittent connections. Replacing the pigtail is often necessary if wiggling the harness triggers the fault. The corresponding ACDelco part number is PT2628.
  Trusted brands: ACDelco, Dorman
  OEM price range: $30-$50
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P1516 — This code is very similar to P1515 and relates to Throttle Actuator Control (TAC) Module performance. They often appear together and point to the same set of problems with the throttle body or its wiring.
• P2135 — This code indicates a correlation error between the two throttle position sensors (TPS 1 and TPS 2). It's another strong indicator that the throttle body assembly is failing internally. TSB #11-06-04-007C addresses this code, though it's more common on later models.
• P1221 / P1271 — These codes relate to the Accelerator Pedal Position (APP) sensor. If they appear with P1515, it could point to a wider issue with the 5-volt reference circuit that is shared between the throttle body and the pedal assembly.

Technical Service Bulletins (TSBs) & Recalls

• TSB #07-06-04-019: Addresses multiple throttle-related DTCs including P1515, P1516, and P2135, often pointing towards issues with the throttle body assembly or wiring.
• TSB #3113 / 10010835: A 2004 bulletin that lists P1515 among a host of other potential PCM-related codes, indicating a known area of concern for GM.

Platform-Specific Known Issues

• The wiring harness for the throttle body is known to become brittle and fail within a few inches of the connector due to engine heat and vibration.
• The GMT360 platform is noted for having grounding issues as it ages, which can manifest as various difficult-to-diagnose electronic problems, including throttle faults.

Mechanic-Grade Diagnostic Values

• Throttle Actuator Control (TAC) Motor Resistance — expected: 2 to 13 Ohms. Failure: A reading below 1 Ohm indicates an internal short; an open circuit (OL) indicates a break in the motor winding. Test is performed between pins E (Yellow) and F (Brown) on the throttle body connector.
• Throttle Position Sensor 1 (TPS1) Resistance — expected: Approximately 3.0 kOhms. Failure: An open circuit (OL) or a reading significantly different from 3 kOhms indicates a faulty sensor. Test is performed between pins C and G on the throttle body.
• Throttle Position Sensor (TPS) Signal Voltage (Scan Tool) — expected: ~0.5V to 0.7V at idle (closed throttle), sweeping smoothly to ~4.5V at wide-open throttle.. Failure: Voltage that is jumpy, erratic, or has dead spots as the throttle plate is slowly opened indicates a worn TPS.
• 5-Volt Reference at Throttle Body Connector — expected: A steady 5 volts at the reference wire pins with key on, engine off.. Failure: Low, absent, or fluctuating voltage points to a wiring issue, a short in another sensor on the same circuit (like the fan clutch), or a PCM fault.

Scan Tool Commands That Help

• Tech 2 / GDS2: Idle Learn Reset / Throttle Learn Reset — This is the official GM method to perform the throttle body relearn procedure after cleaning or replacement. It forces the PCM to find the new minimum throttle position immediately. While a manual procedure often works, this is the most reliable method.
• Tech 2 / GDS2: Throttle Position Graph (Commanded vs. Actual) — This is the definitive way to confirm a failing throttle body. By graphing the PCM's commanded throttle angle against the actual angle reported by the sensors, any lag, jump, or disagreement becomes instantly visible, confirming an internal fault.

Wiring & Ground Locations

• Throttle Body Connector — The 8-pin electrical connector on the throttle body assembly.. This is the single point of failure for all throttle body electrical signals. Pins E (Yellow) and F (Brown) control the motor. The other pins (A, B, C, D, G, H) are for the two TPS sensors, their 5V reference, and ground circuits. Corrosion or broken wires here are a direct cause of P1515.
• G103 — On the cowl in the engine compartment, located above the brake booster.. This is a critical ground point for the Body Control Module (BCM), Instrument Panel Cluster (IPC), and the Data Link Connector (DLC). A poor connection here can cause a wide range of communication and sensor issues, including problems with the throttle control system.
• G104 — On the rear of the driver's side cylinder head.. This is a primary engine ground. Poor contact here can cause unstable sensor readings and intermittent electrical faults for engine management components, including the throttle body.
• G101 — On the lower front of the engine block.. This is a primary ground for the Engine Control Module (ECM). A bad ground here can directly affect the ECM's ability to control the throttle body and interpret its sensor data correctly.

Real Owner Repair Stories

• 2CarPros Forum User (2002 Cadillac Escalade (similar GM platform)) — Reduced Engine Power light, sputtering, and stalling, but only when the ambient temperature was below freezing. Codes P1221 and P1515 appeared.
  ❌ Tried (didn't work) Replacing the Throttle Position Sensor (TPS), Cleaning the throttle body, Replacing the Mass Airflow Sensor, Replacing the throttle body gasket
  ✅ What actually fixed it Replacing the entire throttle body assembly. The problem was internal to the unit and temperature-sensitive. The owner noted that finding a replacement part was difficult, eventually sourcing a remanufactured unit.
• CorvetteForum User (Corvette with similar drive-by-wire system) — P1515 and 'Engine power reduced' mode immediately after reinstalling the air intake assembly.
  ❌ Tried (didn't work) Resetting codes
  ✅ What actually fixed it A foreign object was holding the throttle blade open. A rubber band-like reducer from the aftermarket air intake coupler had fallen into the throttle body and was wedged against the throttle plate, preventing it from closing to the position the PCM expected.
• NHTSA ODI #10318423 (GMC Yukon XL 2500 - similar make) — An owner reported experiencing "Reduced Engine Power" with service engine lights and trouble codes P2135, P0120, P1515, and P0220. Despite spending nearly $2000 on service, the problem remained unresolved.

OEM Part Supersession History

• 12570800 → 19420713 — Standard part lifecycle update, likely incorporating design or material improvements for durability.
  Heads up: The new part number (19420713) is the correct service replacement for the original. The corresponding ACDelco service part is 217-2296.
• 12616437 → 12694872 — Standard part lifecycle update.

Model Year Variations Within This Range

• 2002-2007 vs 2008-2009: The electronic throttle body assembly uses different part numbers. For example, earlier models often use part number 12568580, while later models (2008-2009) may use 12616437 or a successor. While physically similar, the internal electronics or calibrations may be different, making them potentially incompatible. Always verify the part number with the vehicle's VIN.

Diagnostic Flowchart

Other Known Issues on This Vehicle

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

• Cracked Exhaust Manifold 🔴 High — Very common across all years. Often presents as a ticking noise that is loudest on a cold start and may quiet down as the engine warms up. Bolts, especially the rearmost one, are prone to breaking during removal. (Ref: Special Coverage Adjustment 11221A for 2008-2009 models (up to 120,000 miles), but the issue affects all years.)
• Failing Electronic Fan Clutch 🟠 Medium — Common failure item. Symptoms include a loud 'jet engine' or 'roaring' sound from the engine bay (clutch stuck on), overheating at idle (clutch not engaging), or a wobbly fan.
• Faulty Ignition Switch 🟠 Medium — Can cause a no-start/no-crank condition, or allow the engine to shut off but keep accessory power on, draining the battery. The key may get stuck or not turn.
• Bad Water Pump 🟠 Medium — A common failure point as the vehicle ages, often leading to coolant leaks from the sweep hole or bearing failure, which can cause a wobbly fan pulley.
• Fuel Level Sensor Failure 🟡 Low — The fuel gauge becomes erratic or reads empty when the tank is full. This is due to failure of the sensor which is part of the fuel pump module inside the gas tank.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used throttle body from a junkyard can be a cost-effective option, especially if the vehicle is older and the budget is tight. Since this is a common failure item, a used part is a gamble, but can work as a short-to-medium term solution.

Donor-vehicle mileage cap: roughly under 80000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• Look for a donor vehicle with the lowest possible mileage.
• Inspect the throttle bore for excessive carbon buildup; a cleaner unit may indicate better engine health and less wear.
• Check the electrical connector pins for any signs of corrosion, bending, or damage.
• Avoid parts from vehicles that show signs of being in a major front-end collision.

Aftermarket brands forum-validated for this vehicle:

• ACDelco (GM Original Equipment)
• Hitachi
• Bosch
• Standard Motor Products (SMP)

Brands owners have reported issues with on this vehicle:

• Unbranded, 'white-box' parts from online marketplaces may have a higher failure rate. While Dorman offers a version with a redesigned seal, some users prefer to stick with OEM or OEM-supplier brands like Hitachi or Bosch for electronic components.

Real Owner Stories

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

Related OBD-II Codes

• P1516 — Often appears with P1515. Both codes relate to a discrepancy between the PCM's commanded throttle position and the actual position reported by the sensors. TSB #07-06-04-019 specifically groups these codes as having the same root causes (throttle body, wiring).
• P2135 — Throttle/Pedal Position Sensor/Switch 'A'/'B' Voltage Correlation. This code is frequently associated with P1515 because a failing Throttle Position Sensor (TPS) inside the throttle body assembly is a primary cause for both codes. TSB #07-06-04-019 also lists this code.
• P1221 — Throttle Position (TP) Sensor 1-2 Correlation. Mentioned in owner forums as appearing alongside P1515, this code points directly to a fault within the dual sensors of the throttle body, a common failure point on the LL8 engine.
• P0121 — Throttle/Pedal Position Sensor/Switch 'A' Circuit Range/Performance. This is a generic code for a TPS fault. It can be triggered by the same issues that cause P1515, such as a dirty/sticking throttle plate or a failing sensor within the throttle body assembly.
• P2138 — Throttle/Pedal Position Sensor/Switch 'D'/'E' Voltage Correlation. While P1515 is for the throttle body, a failing Accelerator Pedal Position (APP) sensor can send faulty signals to the PCM, causing confusion in the drive-by-wire system. The APP sensor is listed as a final, less common diagnostic step for P1515.

Frequently Asked Questions