2016-2021 Trax & Encore 1.4L AWD 6T40 Transmission: Problems, Identification, and Used Replacement

Identifying the Correct 6T40 AWD Transmission

The automatic transmission in the 2016-2021 Chevrolet Trax and 2016-2021 Buick Encore with All-Wheel Drive (AWD) is the GM Hydra-Matic 6T40. This specific unit was only paired with the 1.4L Turbo LUV engine. Correctly identifying this transmission is critical before purchasing a replacement, as it will not interchange with other versions.

Follow these steps to confirm you have the correct vehicle and transmission:

• Check the VIN: The 8th digit of your vehicle's Vehicle Identification Number (VIN) must be 'B'. This signifies the 1.4L LUV turbocharged engine that was paired with this specific transmission.
• Confirm Drivetrain: This transmission is for All-Wheel Drive (AWD) models only. It cannot be used in a Front-Wheel Drive (FWD) vehicle.
• Check the RPO Code Sticker: Look for a silver sticker labeled "Service Parts Identification" inside the glove box or in the spare tire well. This sticker lists three-character Regular Production Option (RPO) codes for all the options on your vehicle. You should find MNK or MH8, which identifies this specific 6-speed automatic transmission. You may also see FXH, which denotes the final drive ratio.
• Inspect the Transmission Tag: The original transmission has a metal identification tag on the left-hand side of the housing, behind the selector cable and above the pan rail. This tag has a 4-character broadcast code that must be used to match it precisely with a replacement unit.

Common Symptoms of a Failing Transmission

When the 6T40 transmission begins to fail, it can show several distinct symptoms. Recognizing these signs can help you determine if the transmission is the source of the problem.

• Harsh or Delayed Shifting: The vehicle may slam into gear, hesitate, or jerk when shifting, especially between first and second gear or during downshifts.
• Shuddering or Vibration: A feeling similar to driving over rumble strips may occur at steady speeds, particularly during light acceleration. This is often related to the torque converter clutch.
• Loss of Power or Gears: The transmission may slip, where the engine RPMs increase but the vehicle doesn't accelerate properly. In severe cases, it can lose certain gears entirely (like 3rd, 5th, and Reverse) or go into a "limp mode," where it stays in a single gear.
• Check Engine Light: A variety of diagnostic trouble codes (DTCs) can be triggered. A generic P0700 code indicates a general transmission fault, while more specific codes point to solenoids (e.g., P0776) or communication errors (U0101).

Diagnostic Flowchart

Known Issues, Recalls, and Technical Service Bulletins (TSBs)

This transmission family has several well-documented issues. Knowing which problems apply to your model year can aid in diagnosis and repair.

NHTSA Recall 20V668000: Start-Stop Accumulator Bolts

Select 2019-2020 Buick Encore and Chevrolet Trax models with the start-stop system were recalled for missing bolts on the start-stop transmission accumulator. This defect can cause a significant transmission fluid leak, leading to a loss of propulsion and increasing the risk of a crash or fire. The remedy involves dealers inspecting the accumulator and replacing it if bolts are missing. GM's internal number for this recall is N202313440.

DTC U0101: Loss of Communication with TCM

A common issue across all years is the U0101 trouble code, which means the vehicle's main computer has lost communication with the Transmission Control Module (TCM). The TCM (also called a TEHCM) is located inside the transmission on the valve body. Symptoms include a check engine light, the transmission entering a fail-safe "limp mode," and the inability to shift gears. According to GM Technical Service Bulletins PIP4990B, PIP4990C, and PIP4990D, before replacing the expensive TEHCM, a technician should first inspect for loose pins in the main transmission connector, check the TCM ground, and look for any wiring harness rubbing against the engine block.

Broken 3-5-Reverse Wave Plate

A widely known problem in the 6T40 family is the failure of the 3-5-Reverse clutch wave plate. This part is a thin, springy steel plate that applies pressure to a clutch pack. When it breaks, metal fragments can travel throughout the transmission, causing shifting problems or a complete loss of 3rd, 5th, and Reverse gears. While this was more common on pre-2012 Gen 1 units, it has been noted across the family. Later Gen 2 and Gen 3 designs have largely resolved this issue, but it remains a known failure point.

Pressure Regulator Valve Wear (Gen 3 Units)

On Gen 3 units (2015 and newer), wear on the pressure regulator valve is a severe issue. GM technician bulletins advise inspecting the pump cover and replacing a worn pressure regulator valve to prevent a loss of line pressure, which ultimately leads to clutch burn-up.

Axle Bushing Wear & Seal Leaks

Across all years, the driver-side axle seal is prone to leaking. GM service bulletins recommend replacing the seal and inspecting the internal differential case axle support bushing for wear, which often causes the seal failure.

Torque Converter Shudder

Drivers may experience a shudder or vibration during light acceleration at steady speeds. This can be caused by a failing torque converter clutch (TCC) or contaminated transmission fluid. Diagnosing the TCC or addressing fluid contamination is required to fix this moderate severity issue.

What Fails at Every Mileage Band

Under 60,000 Miles

Even at lower mileages, some owner reports mention transmission issues starting as early as 30,000 miles, including slipping, jerking, and hesitation. The 2019-2020 recall (20V668000) for missing start-stop accumulator bolts can cause fluid leaks and loss of propulsion. Failures of the internal TEHCM can cause erratic shifting, and torque converter issues can present as a shudder. Fluid leaks from worn seals or damaged pans are a common early sign of trouble.

• ✓Do during install (required): Replace all external seals (torque converter seal, axle seals).
• ✓Do during install (required): Install a new torque converter; do not reuse the old one.
• ✓Do during install (required): Thoroughly flush the transmission cooler and lines with a dedicated transmission line flushing solvent to remove any debris from the previous unit.
• ✓Do during install (recommended): Perform a transmission fluid and filter service. A fluid change is recommended every 30,000-45,000 miles.
• ✓Do during install (recommended): Inspect the start-stop accumulator on 2019-2020 models to ensure all bolts are present per recall 20V668000.
• ✓Cheap insurance while the engine is out: Inspect the main transmission connector pins to ensure they are secure and the harness is routed away from sharp engine block edges.

60,000 - 100,000 Miles

Torque converter failure becomes more common in this range. Shift flares, slipping, and harsh shifts (especially 2-3) are often linked to valve body wear or solenoid issues. Transmission fluid degradation is a major concern, leading to overheating, slipping, and internal component wear if not serviced. Internal components like clutch packs and transmission bands can begin to wear out.

• ✓Do during install (required): All items from the 'Under 60K' band apply.
• ✓Do during install (required): Perform a full transmission fluid and filter service with DEXRON-VI Full Synthetic ATF (7.4 to 9.5 quarts for a dry fill).
• ✓Do during install (recommended): Consider replacing the internal TEHCM/solenoid body, as they are a frequent failure point and accessible during replacement.
• ✓Do during install (recommended): Inspect and consider replacing the pressure regulator valve, especially on 2015+ Gen 3 units, to prevent low line pressure and clutch burn-up.
• ✓Cheap insurance while the engine is out: Replace the driver-side axle seal and inspect the internal differential case axle support bushing for wear.

100,000 - 150,000 Miles

Internal mechanical failures, such as a broken 3-5-Reverse wave plate, become more likely, leading to a loss of those gears. Wear on internal gears and clutch apply pistons can lead to slipping or failure to engage gears. Overall transmission wear is expected, and the likelihood of needing a replacement versus a repair increases significantly.

• ✓Do during install (required): All items from the previous bands apply.
• ✓Do during install (required): If installing a used unit, it is critical to replace all accessible wear items, including the TEHCM/solenoid body, all seals, and the torque converter.
• ✓Do during install (recommended): A used transmission in this mileage range is a higher risk. A remanufactured unit with component-level updates (e.g., improved valve body, updated clutch packs) is a strongly recommended alternative to a high-mileage used unit.
• ✓Cheap insurance while the engine is out: Inspect and replace output shaft seals between the transmission and the Power Take-Off Unit (PTU), and lubricate the splines with high-pressure grease.

Over 150,000 Miles

At this mileage, the transmission is approaching or has exceeded its expected service life of 150,000 to 200,000 miles. Failures are typically due to cumulative wear and tear on all internal components. Catastrophic failure of hard parts like drums, hubs, or planetary gear sets is highly likely.

• ✓Do during install (required): Installing a used transmission with over 150,000 miles is not recommended. The risk of imminent failure is very high.
• ✓Do during install (recommended): The only recommended course of action at this stage is to install a professionally remanufactured or new transmission assembly.
• ✓Cheap insurance while the engine is out: N/A.

Real Owner Experiences

The following table highlights real-world experiences from owners dealing with 6T40 transmission issues across various mileages.

Transmission Replacement Cost Comparison

The cost to replace a 6T40 transmission varies significantly based on the type of replacement unit you choose. Labor for this job is intensive, typically requiring 7 to 14 hours.

Repair or Replace?

Deciding whether to repair your existing transmission or replace the entire assembly depends heavily on the specific failure mode and the overall cost.

When Replacement is Smarter: Replacement is the better option when internal mechanical failure occurs, such as clutch pack burn-up or gearset damage. When the cost of a specific repair (like a torque converter or wave plate) plus labor approaches or exceeds the cost of a lower-mileage used transmission ($1,200-$2,500 installed), replacing the entire unit is the more economical and reliable long-term choice.

What's Included in a Used Assembly?

When buying a used transmission, it's important to know what parts are typically included versus what you'll need to transfer from your old unit. Based on standards from the Automotive Recyclers Association (ARA), a used assembly's contents are as follows:

Freight & Delivery Reality

A used 6T40 AWD transmission is a heavy, bulky industrial component. It does not ship in a cardboard box via standard parcel services. Verified shipping data shows this assembly weighs approximately 195 lbs and measures roughly 35" x 24" x 19" before crating.

It will ship as Less-Than-Truckload (LTL) freight strapped to a wooden pallet. If you are having it delivered to a residential address or a shop without a loading dock or forklift, you must request a truck with a liftgate to lower the pallet to the ground.

When the freight carrier arrives, you must inspect the transmission for any physical damage (such as cracked aluminum housings or crushed pans) before signing the delivery receipt. Signing without inspecting means you accept the shipment in its current condition, which can void shipping damage claims.

Industry best practice dictates a pre-purchase verification interview with the seller. Be prepared to confirm your VIN's 8th digit is 'B', your vehicle is AWD, your RPO codes include MNK or MH8 and FXH, and provide the 4-character broadcast code from your original transmission's metal tag.

Critical Installation and Programming Gotchas

Installing this transmission is an expert-level task with a DIY difficulty rating of "Expert." It requires special tools, including an engine support fixture, a transmission jack, and subframe alignment tools. There are several potential pitfalls that can cause immediate failure if not done correctly.

After installation and programming, a mandatory Transmission Adaptive Values Learn (Fast Learn) procedure must be performed with a GM MDI/GDS2 or equivalent professional scan tool capable of reading Transmission Fluid Temperature (TFT). This procedure quickly teaches the TEHCM the engagement points of the clutches. Skipping this step will cause harsh shifting and can quickly damage the new transmission.

Other Key Installation Points:

• Fluid Level Check Temperature: This transmission has no dipstick. The fluid level must be checked via a plug on the side of the case ONLY when the transmission fluid is at a specific temperature (strictly between 85°C and 95°C, or 70°C and 80°C depending on your specific GM service manual version), with the engine idling and the vehicle level. Incorrect fluid level will cause damage.
• Flush the Cooler: The transmission fluid cooler and lines must be thoroughly flushed with a dedicated solvent before connecting them to the replacement transmission. Reusing contaminated fluid or lines will send old debris into the new unit, causing it to fail and voiding most warranties.
• AWD Transfer Case (PTU) Alignment: Inspect and replace the output shaft seals between the transmission and the PTU, and lubricate the splines with high-pressure grease during reassembly.

Arrival Inspection for Your Used Transmission

Fluid Color, Smell, and Magnet Inspection

The 6T40 transmission does not have a traditional flat bottom pan. Instead, it uses a side cover and a magnetic drain plug. Follow these steps when your used unit arrives:

• Check the magnetic drain plug: Remove the drain plug located at the bottom of the case. A small amount of fine, gray metal paste is normal. Large metal shavings or chunky pieces mean internal gears are broken. Reject the unit if you see large metal chunks.
• Inspect the fluid: The 6T40 requires Dexron VI transmission fluid. Catch a few drops of fluid on a white paper towel. Good fluid is red or light brown. If the fluid is thick, dark black, or smells like burnt toast, the internal clutches are destroyed.

Converter Hub and Input-Spline Condition

You must inspect the physical connection points before installing the transmission into your Trax or Encore.

• Torque converter hub: This is the smooth metal snout on the back of the torque converter that slides into the transmission pump. Run your fingernail across this metal surface. It must be perfectly smooth. If you feel deep grooves or scratches, the hub will tear the front pump seal and cause a massive fluid leak.
• Input shaft splines: These are the straight metal teeth on the shaft that connect the transmission to the engine. The teeth must be perfectly straight and evenly spaced. If the splines look twisted, rusted, or rounded off at the edges, the transmission will slip or fail to move the vehicle.

Frequently Asked Questions (FAQ)