P0116 on 2017-2023 GMC Acadia 3.6L: Engine Coolant Temperature Sensor Causes and Fixes

What's Unique About the 2017-2023 GMC Acadia

On the 3.6L V6 (LGX) engine in the second-generation Acadia, the primary challenge with a P0116 code is physical access to the components. Both the ECT sensor and the thermostat are located in a very tight spot under the intake manifold, near the firewall. This turns what is a simple part swap on many vehicles into a more labor-intensive job, often requiring removal of the upper intake manifold. Some experienced mechanics and DIYers have developed workarounds to replace the ECT sensor without removing the manifold, but it requires specific tools like a 1/4" drive 15mm socket and extensions, and considerable patience.

Symptoms You May Notice

• Check Engine Light is on
• Temperature gauge on the dashboard is inaccurate (may stay on cold or fluctuate wildly)
• Engine cooling fans run continuously, even when the engine is cold
• Reduced fuel economy
• Heater may not blow hot air
• Engine may hesitate or run rough, especially when first started
• "Service Steering" message and loss of power steering assist
🎬 See why a thermostat failure triggers the steering assist message.

Most Likely Causes

1. Thermostat stuck open 🔴 High Probability The thermostat failing in the open position is a very common failure mode for this part across many GM vehicles with the 3.6L engine. It directly causes the engine to warm up much slower than the ECM expects, triggering a P0116 or, more commonly, a P0128 code.
   How to confirm: Use a scan tool to monitor the coolant temperature in real-time. If the engine takes an excessively long time (e.g., more than 15-20 minutes of driving) to reach operating temperature (around 195-220°F), the thermostat is almost certainly stuck open.
   Typical fix: Replace the thermostat and thermostat housing. Due to the high labor involved, it is highly recommended to replace the ECT sensor at the same time. The system will also need to be drained and refilled with new DEX-COOL coolant.
   Est. part cost: $450-$550
2. Faulty Engine Coolant Temperature (ECT) Sensor 🟡 Medium Probability
   How to confirm: On a cold engine (after sitting for several hours), use a scan tool to compare the ECT reading with the Intake Air Temperature (IAT) reading. They should be within a few degrees of each other. If the ECT reading is illogical (e.g., -40°F or 250°F), the sensor is bad or the circuit is open/shorted. You can also test the sensor's resistance with a multimeter; at 68°F (20°C), it should be between 2,000 and 3,000 ohms.
   Typical fix: Replace the ECT sensor. Given its difficult location, it is often recommended to replace the thermostat at the same time, as the labor overlaps significantly.
   Est. part cost: $20-$60
3. Low Engine Coolant Level ⚪ Low Probability
   How to confirm: Visually inspect the coolant reservoir and radiator (when the engine is cool) to ensure the coolant is at the proper level. An air pocket in the system, caused by a leak or improper filling, can cause the ECT sensor to give erratic readings.
   Typical fix: Top off the coolant with the correct type (DEX-COOL) and bleed the system to remove any air pockets. Investigate and repair the source of the coolant leak.
   Est. part cost: $20-$40 for coolant
4. Wiring or Connector Issue ⚪ Low Probability
   How to confirm: Visually inspect the ECT sensor's electrical connector and wiring harness for signs of corrosion, damage, or loose pins. Wiggle the connector while monitoring the temperature reading on a scan tool to see if it fluctuates.
   Typical fix: Repair the damaged wiring or clean/replace the connector pigtail. Apply dielectric grease to the new connection to prevent future corrosion.
   Est. part cost: $15-$50

Rare But Worth Checking

• ECM Software Glitch: GM has issued technical service bulletins (e.g., 20-NA-167, 17-NA-374) for various models, including some related to the Acadia, where the ECM's diagnostic parameters for setting P0128 or related codes were too sensitive. The fix is to reprogram the ECM with an updated calibration, which must be done at a dealership or a properly equipped independent shop.
• Faulty Engine Control Module (ECM): This is extremely rare. The ECM should only be considered after all other possibilities, including sensor, thermostat, and wiring, have been exhaustively ruled out.

Diagnosis Steps

1. Check for any other stored trouble codes, paying close attention to P0128, which strongly indicates a faulty thermostat.
2. With the engine cold for several hours, use a scan tool to compare the Engine Coolant Temperature (ECT) reading and the Intake Air Temperature (IAT) reading. They should be within 5°F of each other.
3. If the ECT reading is illogical (e.g., -40°F or 250°F), inspect the ECT sensor wiring and connector for damage. If the wiring is good, the sensor has failed.
4. If the initial temperature readings are plausible, start the engine and monitor the ECT data on the scan tool.
5. The temperature should rise steadily to about 195-220°F and then stabilize as the thermostat opens and closes.
6. If the temperature rises very slowly and struggles to reach the normal operating range, the thermostat is almost certainly stuck open and needs to be replaced.
7. If the temperature reading jumps around erratically, the ECT sensor is likely faulty or has a poor electrical connection.
8. Before starting repairs, check the coolant level and condition. Low or contaminated coolant can also cause issues. Ensure there are no air pockets in the system.

Parts You'll Likely Need

• Engine Coolant Thermostat and Housing Assembly (OEM #12677895) — This is a very common cause of P0116 and P0128, as a stuck-open thermostat prevents the engine from warming up correctly. The part for the 3.6L V6 is typically an integrated housing assembly. 🎬 Watch: Step-by-step guide to replacing the thermostat and housing.
  Trusted brands: ACDelco (OEM), Dorman, Gates
  OEM price range: $450-$550
  Aftermarket price range: $150-$300
• Engine Coolant Temperature (ECT) Sensor (OEM #12691559 (Supersedes 12650595)) — The sensor itself can fail, providing incorrect or erratic readings to the ECM. It's a common point of failure for this code and is often replaced with the thermostat as a preventative measure.
  Trusted brands: ACDelco (OEM), Bosch, Delphi, NTK
  OEM price range: $30-$50
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• P0128 — This code specifically means 'Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)', which points directly to a stuck-open thermostat. It is the most common companion code to P0116 on this platform.
• P0115 — This is a general circuit malfunction code for the ECT sensor, which can appear if there's a hard electrical fault.
• P0117 — Indicates the ECT sensor circuit is reading low (short to ground).
• P0118 — Indicates the ECT sensor circuit is reading high (open circuit).

Technical Service Bulletins (TSBs) & Recalls

• 17-NA-374: Power Steering Assist Reduced Message, MIL Illuminated, DTC P111E Set - This TSB notes that an ECM calibration issue can cause a false temperature plausibility code and recommends a software update. (Note: This TSB number is also used for an unrelated diesel emissions topic on other GM vehicles).
• 20-NA-167 / 20-NA-207: While not exclusively for the Acadia, these TSBs highlight a pattern where GM issues ECM software updates to correct overly sensitive diagnostic parameters for codes like P0128 and to address wiring harness interference issues.

Platform-Specific Known Issues

• The location of the thermostat and ECT sensor on the 3.6L V6 is notoriously difficult to access, being tucked under the intake manifold against the firewall. This significantly increases labor time and cost.
• A GM Technical Service Bulletin (17-NA-374) exists for 2017-2018 models for a related code (P111E), which could cause similar symptoms and is resolved with an ECM update.
• Failure to address P0116/P0128 can lead to a 'Service Steering' message and a loss of power steering assist, a documented issue on this platform and its mates.

Mechanic-Grade Diagnostic Values

• ECT Sensor Voltage (Key On, Engine Off, Cold Engine) — expected: ~2.0V - 3.5V. Failure: A reading of ~5.0V indicates an open circuit or unplugged sensor. A reading near 0V indicates a short to ground.
• ECT Sensor Voltage (Engine at Operating Temperature) — expected: ~0.5V - 1.0V. Failure: Voltage that does not decrease as the engine warms up, or decreases erratically.
• ECT Sensor Resistance at 20°C (68°F) — expected: 2000 - 3000 Ohms (Ω). Failure: Resistance significantly outside this range indicates a faulty sensor.
• ECT Sensor Resistance at 90°C (194°F) — expected: 200 - 300 Ohms (Ω). Failure: Resistance that does not drop to this range as the engine warms indicates a faulty sensor.

Scan Tool Commands That Help

• GDS2 (GM Dealer Tool): Compare ECT Sensor 1 data to other temperature sensors (if equipped) — On a cold-soaked engine, all temperature sensors (ECT, IAT, and others on some models) should read within a few degrees of each other. This is a quick plausibility check.
• GDS2 (GM Dealer Tool): Coolant Pump Bleed Procedure — This is a required step after replacing the thermostat or draining the coolant. It runs the necessary pumps and actuators to purge air from the system, which is critical for preventing future codes and overheating.

Wiring & Ground Locations

• ECT Sensor Connector — On the thermostat housing, at the rear of the engine against the firewall, under the intake manifold.. This is the primary connection point for the sensor. Damage, corrosion, or a loose connection here will directly cause P0116, P0117, or P0118.
• Engine Harness Chafe Points — Potential rub-through spots exist where the harness passes over the intake plenum, against valve cover bolts, on the side of the cylinder head, or against the transmission case.. A chafed wire can cause a short or open in the ECT sensor circuit, leading to a P0116 code even if the sensor and thermostat are good. This is a known issue in the GM 3.6L engine family.
• Engine Ground Straps (G103/G105/G108) — Key engine grounds are typically located on the engine block, cylinder heads, and chassis near the front wheel wells.. A poor or corroded engine ground can create a floating voltage reference for all sensors, including the ECT. This can cause skewed or erratic readings that trigger a performance code like P0116.

Real Owner Repair Stories

• YouTube - David Waltermyer (2017 Cadillac XT5 (same LGX engine and platform)) — Check Engine Light with P0128, "Steering Assist is Reduced, Drive with Care" message on the dash.
  ❌ Tried (didn't work) Took to the dealership who quoted $1300 for the repair.
  ✅ What actually fixed it Owner replaced the thermostat (GM #12678779) and ECT sensor (GM #12650595) himself. This cleared both the check engine light and the power steering warning message.
• YouTube - DadsGarage Diagnostic Specialist (GMC Acadia (2nd Gen)) — P0128 code and "Steering Assist is Reduced" message. Temperature gauge reading at the bottom.
  ✅ What actually fixed it A scan tool confirmed the engine temperature would not rise above ~170°F, indicating a stuck-open thermostat. Replacing the thermostat assembly resolved the issue, with post-repair data showing the engine reaching 205°F and stabilizing correctly.

OEM Part Supersession History

• 12650595 → 12691559 — Standard part revision and update by GM.

Model Year Variations Within This Range

• 2017-2018: A specific TSB (17-NA-374) for a related temperature plausibility code (P111E) that causes similar symptoms applies to these early model years and is fixed with an ECM software update.

Diagnostic Flowchart

Other Known Issues on This Vehicle

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

• Shift to Park Message / Shifter Assembly Failure 🔴 High — Very common on 2017-2019 models. A message 'Shift to Park' appears even when the vehicle is in park, preventing it from shutting off and draining the battery. (Ref: Multiple TSBs issued (e.g., PIT5616A, 23-NA-119), but no formal recall. Fix often involves replacing a microswitch, adding a jumper harness, or replacing the entire shifter assembly.)
• Transmission Shudder / Torque Converter Failure 🟠 Medium — Commonly reported around 50,000-80,000 miles. Owners experience shuddering or jerking during light acceleration or when shifting. (Ref: GM has issued TSBs recommending a specific fluid flush procedure. In many cases, the torque converter or entire transmission requires replacement.)
• Timing Chain Stretch (LGX Engine) 🟠 Medium — While significantly improved over the previous generation (LLT/LFX) engines, the LGX can still suffer from timing chain issues, typically at higher mileage (100k+ miles) or if oil change intervals are extended. (Ref: No specific recall for the LGX, but it's a well-documented issue on the GM High Feature V6 family.)
• Carbon Buildup on Intake Valves 🟡 Low — A consequence of Direct Injection (DI) engines. Becomes noticeable after 80,000-100,000 miles, causing reduced performance, rough idle, and potential misfire codes. (Ref: No TSB; this is a maintenance issue inherent to DI engine design. Requires periodic walnut blasting or chemical cleaning service.)
• A/C Condenser Leaks 🟠 Medium — The A/C condenser is susceptible to damage and leaks, leading to poor or no A/C performance.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For this repair, using used parts is generally not recommended due to the high labor cost. However, if the ECM is diagnosed as the fault (which is rare), a used ECM from a reputable salvage yard can be a cost-effective option, but it will require professional programming to the vehicle's VIN.

Donor-vehicle mileage cap: roughly under 60000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For an ECM, ensure the donor vehicle was not in a flood or fire.
• Verify the part number on the used ECM exactly matches the original.
• Ask for a warranty or return period from the salvage yard.

OEM-only on this vehicle (don't cheap out):

• Engine Coolant Thermostat Assembly: Given the extremely high labor involved and the high failure rate of this part, using a new OEM (ACDelco) part is the safest bet to ensure you don't have to do the job again in the near future.

Aftermarket brands forum-validated for this vehicle:

• ECT Sensor: ACDelco (OEM), Bosch, Delphi, NTK.
• Thermostat Assembly: Dorman, Gates (if OEM is unavailable or cost-prohibitive).
• Connector Pigtail: Standard Motor Products (SMP).

Brands owners have reported issues with on this vehicle:

• No-name, unbranded electronic sensors or thermostat assemblies from online marketplaces. The risk of premature failure is too high for a repair this labor-intensive.

Real Owner Stories

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

Related OBD-II Codes

• P0128 — This is the most common companion code; it indicates the engine is warming up slower than the ECM expects, often caused by the same stuck-open thermostat that triggers P0116.
• P111E — A temperature plausibility code mentioned in TSB 17-NA-374 for 2017-2018 models, often caused by ECM calibration issues rather than mechanical failure.
• PIT5616A — While not an OBD-II code, this TSB relates to the 'Shift to Park' failure, another high-severity known issue for the 2017-2019 Acadia.
• P0117 — Related to the ECT sensor circuit; may appear if the sensor or wiring is shorted, similar to the wiring/connector issues described for P0116.
• P0118 — Related to the ECT sensor circuit; may appear if the circuit is open, which is a diagnostic possibility when the ECT reading is an illogical -40°F.

Frequently Asked Questions