P1516 on 2005-2009 GMC Envoy: Reduced Power, Causes, and Fixes

What's Unique About the 2005-2009 Gmc ENVOY

The GMT360 platform, which includes the GMC Envoy, Chevrolet Trailblazer, Buick Rainier, and Isuzu Ascender, is well-known for issues related to the electronic throttle body, particularly on the 4.2L LL8 inline-six engine. Carbon buildup is a very frequent problem that impedes the movement of the throttle plate, leading to this code. Additionally, as these vehicles age, the wiring harness near the throttle body connector can become brittle and break internally from heat and vibration, or key grounding points can corrode, leading to intermittent electrical faults that are sometimes hard to diagnose.

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 dash 🎬 Watch: 5 common causes and fixes for the P1516 code.board
• Check Engine Light is on
• Little to no throttle response or poor acceleration
• Engine stalling or rough idle
• Idle surge, especially after cleaning the throttle body without a relearn
• Harsh or erratic transmission shifting
• Cruise control may not work

Most Likely Causes

1. Dirty Throttle Body 🔴 High Probability → Shop Fuel Injection Throttle Body Carbon and oil vapor from the crankcase ventilation (PCV) system build up around the throttle plate, restricting its movement. The computer tries to compensate for the blockage, but eventually, the physical restriction causes a mismatch between the commanded and actual throttle position, triggering the code.
   How to confirm: Visually inspect the throttle body bore and plate for a thick black coating of carbon after removing the air intake tube.
   Typical fix: Remove the throttle body and clean it thoroughly with a dedicated throttle body cleaner and a soft brush or rag. Cleaning it 🎬 Watch: Step-by-step guide to cleaning your throttle body correctly. while still on the vehicle is often insufficient.
   Est. part cost: $5-$15
2. Failing Throttle Body Assembly 🟡 Medium Probability → Shop Fuel Injection Throttle Body The internal electronic components, such as the throttle position sensor (TPS) or the actuator motor, can wear out over time, causing dead spots, erratic behavior, or a complete failure of the actuator.
   How to confirm: After cleaning, if the code persists, use a scan tool to graph the commanded vs. actual throttle position (TPS1 and TPS2). Jumps, flat spots, or a significant lag in the data as you slowly press the accelerator pedal indicate a failing sensor or motor.
   Typical fix: Replace the entire throttle body assembly. The sensors and motor are typically integrated and not sold separately for this platform. 🎬 See this walkthrough on troubleshooting and replacing the assembly.
   Est. part cost: $150-$400
3. Wiring or Connector Issues 🟡 Medium Probability Wires in the harness near the throttle body connector can break internally from years of engine vibration and heat cycles. The break is often within an inch or two of the connector itself. The connector terminals can also become loose or corroded.
   How to confirm: With the engine running, carefully wiggle the wiring harness leading to the throttle body to see if it triggers the fault or causes a change in idle. Visually inspect the connector for corrosion, backed-out pins, or damage. Check for continuity on each wire.
   Typical fix: Repair the broken wire(s) or replace the throttle body connector pigtail.
   Est. part cost: $15-$50
4. Poor Ground Connection ⚪ Low Probability Several key engine and chassis grounds can become loose or corroded, causing intermittent and difficult-to-diagnose electrical issues for the TAC system. This is a common fix for owners who have replaced parts but still have the code return intermittently.
   How to confirm: Locate, remove, clean, and tighten all major ground straps and connections, especially those on the engine block, cylinder head, and firewall. A common trouble spot is a ground on the back of the cylinder head.
   Typical fix: Clean the contact surfaces to bare metal and secure the ground connections. Some owners have permanently solved persistent issues by adding a redundant ground wire from the firewall to the engine block and another from the TAC module to the firewall.
   Est. part cost: $0-$20

Rare But Worth Checking

• Throttle Body Icing: → Shop Fuel Injection Throttle Body As mentioned in TSB #PIP4573B, in ambient temperatures around -10°F (-23°C), moisture from the PCV system can freeze inside the throttle body, causing the plate to stick during a cold start and triggering P1516.
• Failing Powertrain Control Module (PCM): This is very rare, but if all other components and wiring have been verified as good, the PCM itself could have a faulty driver circuit for the TAC system. This should be the last resort after all other possibilities are exhausted.

Diagnosis Steps

1. Check for other stored trouble codes that could provide more context, especially P2135, P2101, or P2176.
2. Visually inspect the throttle body for heavy carbon buildup. This is the most common cause and the first step.
3. If dirty, remove the throttle body from the intake manifold for a thorough cleaning. Spraying cleaner while it's on the vehicle is often insufficient and can damage sensors.
4. After cleaning and reinstalling, perform a throttle relearn procedure. A common method is to disconnect the battery for 30 minutes, reconnect it, turn the key to 'ON' (without starting) for a few minutes, then start the engine and let it idle without touching the gas pedal for 8-10 minutes until the idle speed settles.
5. If the code returns, inspect the wiring harness and connector at the throttle body. Check for chafed wires, loose pins, or corrosion. Carefully wiggle the harness with the engine running to see if the fault can be duplicated.
6. Using a capable scan tool, monitor the live data for Commanded Throttle Position vs. Actual Throttle Position (TPS1 and TPS2). The values should track each other closely as the pedal is pressed. Any deviation, dropouts, or spikes points to a faulty throttle body.
7. Check and clean the main engine and body ground points. Key locations include grounds G103 and G104 on the engine block/head and firewall connections. Consider adding redundant ground wires if the problem is intermittent and other fixes have failed.
8. If all other steps fail to resolve the issue, replacement of the throttle body assembly is the most likely solution.

Parts You'll Likely Need

• Throttle Body Assembly (OEM #12570800) — This is the most common repair if cleaning does not solve the problem. The internal sensors and motor wear out and are not serviced separately.
  Trusted brands: ACDelco, Hitachi (ETB0024), Dorman
  OEM price range: $250-$400
  Aftermarket price range: $120-$250
• Throttle Body Cleaner — This is the first and most crucial step in diagnosing and fixing P1516, as carbon buildup is the leading cause.
  Trusted brands: CRC, Berryman, Gumout
  OEM price range: $10-$15
  Aftermarket price range: $5-$10
• Throttle Body Connector Pigtail (OEM #ACDelco PT2628) — If the wiring is found to be broken at the connector, replacing the pigtail is the standard repair. This is an 8-pin connector.
  Trusted brands: ACDelco, Dorman, Michigan Motorsports (G2300)
  OEM price range: $30-$50
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P2101 — This code specifically relates to 'Control Module Throttle Actuator Control Motor Circuit Range/Performance', directly pointing to an issue with the motor inside the throttle body. It is often cited alongside P1516 in TSBs and diagnostic reports.
• P2119 — Indicates 'Throttle Closed Position Performance', often set when carbon buildup prevents the throttle plate from closing properly.
• P2176 — This code, 'Throttle Actuator Control System - Idle Position Not Learned', is often triggered after cleaning or replacing the throttle body if a relearn procedure is not performed or is unsuccessful. TSB PIP4578B specifically addresses this scenario.
• P2135 — This code for 'Throttle/Pedal Position Sensor/Switch A/B Voltage Correlation' is a very common companion to P1516, as the PCM is seeing conflicting data from the two TPS sensors within the throttle body assembly.
• P0121 — Relates to 'Throttle/Pedal Position Sensor 'A' Circuit Range/Performance', indicating the data from one of the TPS sensors is not what the PCM expects, which is a direct companion to P1516.

Technical Service Bulletins (TSBs) & Recalls

• PIP4578B: Mentions P1516 in relation to idle surge and reduced power mode on high mileage vehicles due to carbon buildup. Stresses the importance of performing an idle learn reset after cleaning.
• PIP4573B: Connects P1516 to cold start issues in freezing temperatures and potential throttle body icing.

Platform-Specific Known Issues

• TSB #PIP4578B notes that P1516 can occur on high-mileage vehicles in conjunction with an idle surge, and explicitly states that after cleaning the throttle body, an 'idle learn reset' must be performed with a scan tool to prevent the issue from returning.
• TSB #PIP4573B points to potential throttle body icing as a cause for this code during extremely cold starts (-10°F / -23°C), where moisture can freeze and cause the throttle plate to stick.

Mechanic-Grade Diagnostic Values

• Scan Tool Parameter: TP Sensor 1 and 2 Agree/Disagree — expected: Should display 'Agree' throughout pedal travel.. Failure: Parameter switches to 'Disagree' at any point during pedal movement.
• Scan Tool Live Data: Commanded vs. Actual Throttle Position — expected: Actual position should closely mirror commanded position with minimal lag.. Failure: Actual position lags, jumps, sticks, or does not match commanded position.
• Throttle Body Connector Wire Wiggle Test — expected: No change in engine idle or stored codes when wiggling individual wires.. Failure: Wiggling wires near the connector causes the engine to stumble, triggers limp mode, or sets a DTC.
• TAC Module Voltage Reference — expected: Reference voltage should be stable and match specifications (e.g., 12V or 5V).. Failure: A significant voltage drop, such as reading 8 volts on a 12-volt reference circuit, indicates a wiring or module issue.

Scan Tool Commands That Help

• Tech2/GDS2 or equivalent: Idle Relearn / Throttle Relearn — This function must be used after replacing the throttle body or cleaning it to reset the learned idle and throttle position values in the PCM. Failure to do so can cause a high idle, stalling, or a P2176 code.
• Tech2/GDS2 or equivalent: Throttle Position Sweep Test (Bidirectional Control) — To test the throttle motor and sensors without the engine running. The command sweeps the throttle plate from closed to wide open, allowing you to watch live data for any glitches, sticking points, or sensor dropouts.

Wiring & Ground Locations

• G103 and G104 — Main engine grounds. G104 is often located on the back of the driver's side cylinder head. G103 can be on the engine block or cowl area. A wiring diagram confirms G103 is on the left front of the engine compartment on the radiator support for some models.. These are primary grounds for the engine control system. Corrosion or looseness at these points can cause intermittent voltage drops to the PCM and TAC system, leading to P1516 and other codes. They are a known trouble spot for persistent, hard-to-diagnose issues.
• Throttle Body Harness — The wiring harness within 1-4 inches of the 8-pin connector at the throttle body.. This area is prone to internal wire breaks from heat and vibration. The insulation may look fine, but the copper strands inside can be broken, causing an intermittent open circuit that triggers P1516.
• Redundant/Added Grounds — User-installed 10-gauge wires. Common paths are: 1) Firewall to a bolt on the engine block/head. 2) A mounting bolt on the TAC module body to the firewall. 3) Battery negative terminal directly to the TAC module body.. This is often the final fix when replacing parts does not work. It corrects for a poor or high-resistance factory ground path that may be difficult to locate, providing a solid, reliable ground for the entire throttle control system.

Real Owner Repair Stories

• Tahoe Yukon Forum (2005 Yukon Denali (GMT800 platform, similar electronics)) — Reduced Engine Power message, multiple codes including P1516, P1518, P2108, P2135. Problem would return after a few days or after hitting bumps.
  ❌ Tried (didn't work) Previous owner had replaced the throttle body pigtail connector.
  ✅ What actually fixed it The owner added multiple new ground wires using 10-gauge copper wire. Key additions were from the firewall to the engine block, from the TAC module to the firewall, and several others to ensure a robust ground path. The problem was permanently resolved.
• YouTube user 777boatman (2004 GMC 2500 6.0L (Similar TAC system)) — Intermittent 'Reduced Power' mode with codes U0107 and P1516. Problem would return anywhere from a day to a month after repairs.
  ❌ Tried (didn't work) Replacing the throttle body, Replacing the throttle body harness pigtail, Replacing the TAC module
  ✅ What actually fixed it The owner created a new, dedicated grounding system. He ran an 8-gauge wire from the battery negative terminal to a sanded-clean spot on the TAC module body, then from the module to the firewall, to the alternator mounting bracket, and to the computer case, creating a complete, redundant ground path. The issue never returned.
• GM Forum (2006 GMC Sierra 2500HD 6.0L) — Reduced Engine Power mode with P1516, sometimes with P2135 and P0220. The issue would trigger when coming to a stop or at idle.
  ❌ Tried (didn't work) Cleaning the throttle body, Replacing the throttle position sensor on the side of the TB, Performing a throttle relearn procedure
  ✅ What actually fixed it Replacing the entire throttle body assembly finally resolved the codes and the limp mode issue.

When the Usual Fixes Don't Work

• While cleaning or replacing the throttle body is the most common fix, a significant number of owners report that these steps do not solve intermittent P1516 issues. In these cases, the problem is almost always traced back to a faulty ground system. The factory grounds can become resistive over time, even if they look clean. The definitive, long-term solution for many who have already replaced parts has been to add new, redundant ground straps from the battery negative to the TAC module, from the firewall to the engine block, and between other key points to ensure the throttle control system has a consistently stable ground reference.

OEM Part Supersession History

• 12570800 → 19420713 — Standard part lifecycle update by the manufacturer.

Model Year Variations Within This Range

• 2005-2009: The throttle body for the 4.2L I6 engine is a different part than the one used on the 5.3L V8 engine available in some Envoy models. Ensure the correct part is ordered for the specific engine in the vehicle.