P2797 on 2023-2024 Subaru Outback: Auxiliary Transmission Pump Fault Causes and Fixes

What's Unique About the 2023-2024 Subaru OUTBACK

What's unique to this platform is that Subaru has officially acknowledged a primary cause for P2797 is a software logic error, not a widespread hardware failure. A Technical Service Bulletin (TSB 16-145-23R) was issued specifically to reprogram the Transmission Control Module (TCM) because it was falsely detecting a pump failure during auto start-stop cycles. The bulletin explains this is due to a logic error related to forward clutch engagement during engine restart. This means that for many owners, the fix is a relatively simple and quick software update at the dealership rather than an expensive and invasive transmission repair.

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 is on
• Transmission warning light illuminated on the dash
• A shock or surge felt when the engine restarts via the auto start-stop feature
🎬 Watch: A quick guide to how Subaru's auto start-stop works.
• Auto start-stop system may become inoperative
• Delayed or harsh gear shifts
• Vehicle may not allow selection of certain driving modes, like Sport mode
• Flashing 'I' (Intelligent) drive mode indicator

Most Likely Causes

1. Transmission Control Module (TCM) Software Error 🔴 High Probability → Shop Transmission Assembly Subaru has identified a logic error in the TCM software that can falsely trigger DTC P2797 during auto start-stop events due to incorrect torque transfer from the forward clutch system.
   How to confirm: A Subaru dealer can check the vehicle's service history and current TCM software version against TSB 16-145-23R to see if the update applies and has not yet been performed.
   Typical fix: Reprogramming the TCM with updated software files provided by Subaru. The TSB 🎬 See how a technician performs a TCM update and relearn. explicitly states that a CVT Learning procedure is not needed after this reprogram.
   Est. part cost: $0 (if under warranty)
2. Failed Auxiliary Transmission Fluid Pump ⚪ Low Probability → Shop Transmission Assembly While less common than the software issue on these models, the electric pump can still fail due to an internal motor or mechanical fault. This is a known failure point on other vehicles that use an auxiliary pump for their start-stop systems.
   How to confirm: If the TCM reprogram does not fix the code, a technician will need to test the pump's electrical circuit for power and ground, and may use a scan tool to command the pump on to verify its operation.
   Typical fix: Replace the auxiliary transmission fluid pump and potentially its wiring harness.
   Est. part cost: $250-$500
3. Wiring or Connector Issue ⚪ Low Probability Not specific to this vehicle, but any vehicle can experience issues with damaged wiring harnesses or corroded/loose connectors leading to a component.
   How to confirm: Visual inspection of the wiring harness leading to the auxiliary transmission pump and checking for continuity and voltage at the connector.
   Typical fix: Repairing the damaged wire or cleaning/replacing the connector.
   Est. part cost: $5-$50

Diagnosis Steps

1. Connect an OBD-II scanner and confirm the presence of P2797 and any other codes (like P0700).
2. Contact a Subaru dealership and reference Technical Service Bulletin (TSB) #16-145-23R. Inquire if the software update has been applied to your vehicle's VIN.
3. If the update has not been performed, have the dealer reprogram the Transmission Control Module (TCM) as per the TSB. Note: The TSB states there is no need to perform a CVT Learning procedure after this specific reprogram.
4. Clear the codes and perform a test drive, making sure to allow the auto start-stop system to engage several times to confirm the fix.
5. If the code returns after the reprogram, the fault is likely with the hardware. Proceed with diagnosing the auxiliary pump circuit.
6. Inspect the wiring and connector at the auxiliary transmission fluid pump for any signs of damage, corrosion, or loose connections.
7. Using a multimeter, check for proper voltage and ground at the pump's connector when it is commanded on. Expect to see battery voltage (approx. 12V) and near 0 Ω resistance to ground.
8. If possible, use a bi-directional scan tool to command the auxiliary pump to activate. Listen for pump operation.
9. If the wiring provides correct power and ground, but the pump does not run or runs poorly, the auxiliary transmission fluid pump has failed and needs to be replaced.

Parts You'll Likely Need

• Transmission Control Module (TCM) Reprogram — This is the most common fix identified by Subaru in TSB 16-145-23R to correct a software logic flaw that falsely triggers code P2797.
  Trusted brands: Subaru (Dealer Service)
  OEM price range: $0 - $200 (May be covered by warranty)
  Aftermarket price range: N/A
• Auxiliary Transmission Fluid Pump (OEM #30726AA020 (for 2020-21 models, verify for 23-24)) — If the software fix does not work, the pump itself is the next most likely point of failure, either electrically or mechanically.
  Trusted brands: Subaru OE
  OEM price range: $250 - $500
  Aftermarket price range: $150 - $350

Related Codes That Often Appear With This One

• P0700 — P0700 is a generic code that simply means the Transmission Control Module (TCM) has stored a fault code. It acts as a 'check engine light' for the transmission, and it will almost always be present when a specific transmission code like P2797 is logged.

Technical Service Bulletins (TSBs) & Recalls

• 16-145-23R: Announces new reprogramming files for the Transmission Control Module (TCM) to address cases of DTC P2797. It clarifies the cause is a false detection due to incorrect torque transfer during an engine restart from auto start-stop. It applies to 2023-2024 Legacy and Outback 2.4L models and states a CVT relearn is not necessary post-update.

Platform-Specific Known Issues

• A known software issue causes the TCM to incorrectly detect a failure of the auxiliary transmission pump during auto start-stop restarts, which can cause a surge or shock. This is addressed by TSB 16-145-23R.
• Owners on forums frequently report this code appears immediately after an auto start-stop event in city driving or a drive-through.
🎬 Tired of the system? See how to install a start-stop eliminator.
• Technicians on forums have warned that dealers may misdiagnose this issue and recommend a full transmission replacement, when in fact a simple software update or relearn procedure is the correct fix specified by Subaru.

Mechanic-Grade Diagnostic Values

• Voltage at auxiliary pump power feed wire — expected: 12 V ± 0.5 V (with ignition ON). Failure: Voltage significantly below 12V indicates a wiring or fuse issue.
• Resistance of auxiliary pump ground wire to chassis — expected: 0 Ω. Failure: Any significant resistance indicates a bad ground connection.
• Power supply voltage during TCM reprogramming — expected: A steady 13.5 Volts. Failure: Voltage rising above 14V will cause the procedure to abort. This indicates a vehicle battery issue that must be resolved first.

Scan Tool Commands That Help

• Subaru Select Monitor (SSM) or equivalent professional scan tool: Auxiliary Pump Activation Test — Use this bidirectional control if the P2797 code returns after the TSB reprogram. It commands the pump to turn on, allowing a technician to audibly check for operation and verify the integrity of the pump motor itself.
• Subaru Select Monitor (SSM): CVT Shift Relearn Procedure — A technician on a forum reported this 20-minute procedure can resolve an incorrect P2797 trigger when the official diagnostic tree points to a full transmission replacement. This may be a viable step to try before committing to major hardware replacement.

Wiring & Ground Locations

• TCM (Transmission Control Module) — Generally located under the driver's side dashboard.. The TCM is the module that stores code P2797 and requires the software update (TSB 16-145-23R) to fix the logic error.
• Main Transmission Harness Connectors (T4/B11) — On TR690 CVTs, a key grey connector (often designated T4 or B11) is located on the top of the transmission case.. This connector houses pins for multiple transmission sensors and solenoids. While not specific to the auxiliary pump, checking for corrosion or damage here is a key step in any transmission electrical diagnosis.
• ECU Ground Point — A critical ground point for the ECU on many Subarus is located on the top of the intake manifold.. A poor ground at this location can cause a wide range of difficult-to-diagnose electrical issues. While not a direct cause of P2797, ensuring this ground is clean and secure is a foundational step in any electrical troubleshooting.

Real Owner Repair Stories

• Reddit user iAcredeaux in r/Subaru_Outback (2023 Outback Limited XT) — Transmission 'failure' related to the auto start/stop feature.
  ❌ Tried (didn't work) The user did not report trying other fixes.
  ✅ What actually fixed it The dealership replaced the entire transmission under warranty. The user notes the issue was a failed auxiliary pump, which is inconveniently located inside the transmission, making replacement of the whole unit an easier option for the dealer.

"I Checked Everything" — The Actual Cause

• For code P2797 on this platform, it's common for all electrical tests of the auxiliary pump circuit to pass, showing good voltage, ground, and continuity. The actual cause, as identified in TSB 16-145-23R, is not a hardware or wiring failure but a software logic error in the TCM that falsely detects a pump problem during an engine restart. Therefore, a 'clean' bill of health on the physical circuit is expected when the software is the root cause.

OEM Part Supersession History

• 30919AJ71A (Software Part Number) → Z1FEF63100L (New CID File) — The new software file contains enhanced logic to optimize forward clutch engagement during auto start-stop restarts, preventing the false detection of a pump failure.
  Heads up: This is a software update for the TCM on the 2023-2024 Outback 2.4L Turbo, not a physical part replacement.

Model Year Variations Within This Range

• 2023-2024: The P2797 issue addressed by TSB 16-145-23R is specific to models equipped with the 2.4L Turbo engine and the TR690 Lineartronic CVT. Models with the 2.5L naturally aspirated engine are not listed in this specific bulletin.