P0977 on 2020-2023 Subaru Legacy: Shift Solenoid 'B' Circuit High Causes and Fixes

What's Unique About the 2020-2023 Subaru LEGACY

The 2020-2023 Subaru Legacy uses a TR580 CVT and is highly susceptible to valve body failures. Subaru has acknowledged this pattern by issuing Technical Service Bulletin 16-149-25, which directly links P0977 and other related codes 🎬 See this breakdown of P0977 engine code causes and symptoms. to a specific design flaw. The TSB explains that condensation can accumulate inside the transmission cover and seep into the valve body's main harness connector, causing a short to power and triggering high circuit voltage codes. This makes the valve body the primary suspect, far more so than on other vehicles where wiring or fluid issues might be more common.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Check Engine Light illuminated
• AT OIL TEMP warning light flashing
• Multiple warning lights on the dashboard simultaneously (often called the 'Christmas tree' effect), including ABS, Eyesight, and Vehicle Dynamics Control
• Harsh or delayed gear shifts
• Vehicle stuck in one gear (limp mode)
• Rough or delayed acceleration
• Hesitation when accelerating from a stop
• Reduced fuel efficiency

Most Likely Causes

1. Defective Transmission Valve Body 🔴 High Probability → Shop Transmission Valve Body Subaru TSB #16-149-25 directly addresses this failure. It states that condensation can accumulate within the transmission cover and seep into the harness connector for the CVT control valve, causing high circuit voltage codes like P0977. This is a widely documented issue on this platform and its mates.
   How to confirm: A technician will follow the diagnostic steps outlined in TSB 16-149-25, which involves checking the harness connector for moisture and testing resistance and voltage between the TCM and the valve body. If the wiring tests good, the valve body assembly is the confirmed point of failure.
   Typical fix: Replacement of the entire transmission valve body with the updated part (Control Valve Kit P/N: 31825AA102) and installation of the new connector cover (P/N: 29174AA010) to prevent future moisture intrusion, as specified by Subaru. A TCM relearn procedure is required after replacement.
   Est. part cost: $700-$900 for an OEM valve body assembly.
2. Damaged Wiring or Connectors ⚪ Low Probability While the TSB points to moisture in the connector as the primary cause, physical damage to the harness is less common but possible.
   How to confirm: Visually inspect the transmission wiring harness for any signs of chafing, rodent damage, or corrosion, particularly at the main connector to the transmission. The TSB diagnostic procedure also includes specific resistance and voltage checks to isolate a harness fault from an internal valve body fault.
   Typical fix: Repair or replace the damaged section of the wiring harness. For the 2020-23 Legacy, the transmission harness part number is 24031AA893.
   Est. part cost: $50-$500 depending on the extent of the damage.
3. Low or Contaminated Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly Unlikely on a newer vehicle unless there is a leak or improper service. However, extremely dirty or low fluid can cause erratic solenoid operation and other transmission issues.
   How to confirm: Check the level and condition of the CVT fluid according to the manufacturer's procedure. The fluid should be at the correct level and not appear dark, burnt, or contain excessive metal particles.
   Typical fix: Perform a CVT fluid drain and fill using the correct Subaru High-Torque CVT fluid. If the fluid is heavily contaminated, it may indicate a more severe internal transmission problem beyond the valve body.
   Est. part cost: $150-$250 for fluid.

Rare But Worth Checking

• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly This is very rare. The TCM is the computer that controls the transmission. It should only be considered for replacement after all other possibilities, especially the valve body and wiring, have been exhaustively ruled out per the TSB diagnostic flow.

Diagnosis Steps

1. Verify the code with a quality OBD-II scanner.
2. Check for any related Technical Service Bulletins (TSBs). For this code, TSB 16-149-25 is the primary document.
3. Check if the vehicle is covered under the Subaru CVT Warranty Extension (10 years/100,000 miles) by contacting a dealer or checking Subarunet.
4. Follow the diagnostic procedure in TSB 16-149-25. This begins with inspecting the transmission harness connector for moisture or damage.
5. Measure the resistance and voltage of the shift solenoid 'B' circuit at the TCM connector to isolate the issue to the wiring or the valve body itself. The TSB provides specific pins and expected values.
6. If wiring and connections are confirmed to be good, the fault lies within the transmission valve body.
7. Replace the valve body assembly with the updated OEM part (31825AA102) and the new connector cover (29174AA010) as specified in the TSB.
8. Refill with the correct Subaru High-Torque CVT fluid.
9. Perform the TCM relearn procedure using a compatible scan tool.

Parts You'll Likely Need

• Transmission Valve Body Assembly (Control Valve Kit) (OEM #31825AA102) — This is the most common cause of P0977 on this vehicle, as documented by Subaru's own TSB. The solenoids are integrated, and the assembly is an updated design to prevent recurrence.
  Trusted brands: Subaru (OEM)
  OEM price range: $700-$900
  Aftermarket price range: Not Recommended
• Connector Cover (OEM #29174AA010) — A new, redesigned cover is required by TSB 16-149-25 to prevent the moisture intrusion that causes the original failure.
  Trusted brands: Subaru (OEM)
  OEM price range: $10-$20
  Aftermarket price range: N/A
• Subaru High-Torque CVT Fluid — The correct fluid must be used when replacing the valve body and refilling the transmission.
  Trusted brands: Subaru (OEM)
  OEM price range: $20-$30 per quart
  Aftermarket price range: Not Recommended

Related Codes That Often Appear With This One

• P0974 — Shift Solenoid 'A' Control Circuit High. This code is listed in the same TSB (16-149-25) and points to the same root cause: a faulty valve body or connector issue.
• P2763 — Torque Converter Clutch Pressure Control Solenoid Control Circuit High. Also listed in TSB 16-149-25, indicating a failure within the same valve body assembly.
• P2721 — Pressure Control Solenoid 'D' Control Circuit High. Also listed in TSB 16-149-25, pointing to the common valve body failure.
• P0700 — Transmission Control System Malfunction. This is a generic code indicating that the TCM has stored a specific fault code, such as P0977. It will almost always be present alongside the specific solenoid code.

Technical Service Bulletins (TSBs) & Recalls

• 16-149-25: Describes the cause (moisture in connector) and fix (new valve body and connector cover) for DTCs P2721, P2763, P0974, & P0977 on 2020-23 Legacy/Outback and other models.
• 16-155-25R: Details the CVT Warranty Extension to 10 years/100,000 miles for applicable models, which may cover the P0977 valve body repair.

Platform-Specific Known Issues

• Subaru CVT Warranty Extension: Subaru has extended the CVT warranty for many models, including the 2020 Legacy, to 10 years / 100,000 miles, whichever comes first. Owners experiencing P0977 should immediately check with a Subaru dealer to see if their specific vehicle's VIN is covered under this warranty extension (bulletin 16-155-25R), as the valve body replacement could be performed at no cost.
• TSB 16-149-25: This TSB is the definitive guide for this issue. It was released to address P0977 and related codes by providing an updated valve body and a new connector cover to prevent moisture intrusion, which was identified as the root cause.

Mechanic-Grade Diagnostic Values

• Shift Solenoid 'B' (Primary Up/Down) Resistance — expected: 10.0 - 13.5 Ohms. Failure: A reading that is significantly higher (high resistance), lower, or open (infinite resistance) indicates a failed solenoid coil.
• AWD Transfer Clutch Solenoid Resistance — expected: 2.0 - 4.5 Ohms. Failure: This solenoid has a different specification; confusing it with a shift solenoid could lead to misdiagnosis.
• Lock-Up Duty Solenoid Resistance — expected: 10.0 - 13.5 Ohms (approx. 12 Ohms at 68°F). Failure: An open circuit, especially when the transmission is hot, is a common failure mode for this solenoid.

Scan Tool Commands That Help

• Subaru Select Monitor (SSM) or equivalent professional scan tool (e.g., Autel): Clear Memory / Learned Value Reset — This function should be performed after replacing the valve body to erase the old adaptive learning values stored by the TCM.
• Subaru Select Monitor (SSM) or equivalent professional scan tool: AT Learning (Compulsory Learning Mode) — This is a specific, guided procedure required after clearing the memory to allow the TCM to learn the hydraulic characteristics of the new valve body solenoids. It typically involves running the vehicle on a lift with wheels off the ground and following on-screen prompts.
• Subaru Select Monitor (SSM) or equivalent professional scan tool: Active Test — This diagnostic function allows a technician to command individual solenoids ON and OFF to verify their operation, which can help confirm if a solenoid is stuck before parts are replaced.

Wiring & Ground Locations

• TCM Connector — The Transmission Control Module (TCM) is located in the engine bay. Diagnostics for solenoid circuits are performed by back-probing the appropriate pins at this connector.. TSB 16-149-25 and 16-131-20R instruct technicians to measure voltage and resistance between the TCM connector and the transmission body/connector to determine if the fault is in the external wiring harness or internal to the transmission (valve body).
• Shift Solenoid 'B' (Primary Down) Wires — Inside the transmission, connected to the valve body.. Knowing the wire colors can assist in correctly identifying the circuit during testing. For the Gen 2 TR580, the Primary Down solenoid wires are typically Gray and Blue.
• Shift Solenoid 'A' (Primary Up) Wires — Inside the transmission, connected to the valve body.. Often fails along with Solenoid 'B'. The wires are typically Black and Purple.

Real Owner Repair Stories

• Synthesized from multiple YouTube repair videos (2013 Subaru Legacy (similar TR580 transmission and repair process)) — Check Engine Light, flashing AT Temp light, and a lock-up solenoid related code (P2764).
  ❌ Tried (didn't work) Initial online research pointed towards a solenoid failure, but individual solenoids are not the recommended fix.
  ✅ What actually fixed it The entire transmission valve body was replaced with a refurbished unit. After the repair and reconnecting the battery, the vehicle was test-driven. It initially 'bucked a little bit' as it seemed to go through an automatic relearn process, and then began shifting smoothly after about half a mile. All warning lights went away.

OEM Part Supersession History

• Unknown → 31825AA102 — The original valve body design was susceptible to moisture intrusion at the main connector. Part number 31825AA102 is the updated assembly designed to prevent this failure, especially when installed with the new connector cover (29174AA010).
  Heads up: Using a salvaged or older-design valve body will likely lead to a repeat failure.

Model Year Variations Within This Range

• 2020-2021: These model years are also susceptible to a separate, well-documented failure of the Thermo Control Valve (TCV). This issue throws different codes (e.g., P26A3) but produces similar symptoms, including multiple warning lights (Eyesight disabled, Check Engine) and the vehicle entering a fail-safe mode. Owners seeing these symptoms should verify the specific DTCs to differentiate between the P0977 valve body issue and the TCV issue, which has its own warranty extension (WMM-23).