P0720 Code: Subaru Forester (2019-2024) - Causes, Symptoms & Fixes

CODE MEANING AND SEVERITY

• Code: P0720 — Output Speed Sensor Circuit Malfunction (Output Shaft Speed Sensor)
• What it means: The transmission control module (TCM) or the engine control module (ECM) has detected an abnormal signal from the Output Shaft Speed (OSS) sensor, or a fault in the OSS sensor circuit. The OSS sensor provides the transmission with the shaft speed information the TCM uses to control shifting and lockup. A fault can be intermittent or permanent.
• Severity: Moderate to high. If the OSS signal is unavailable or inconsistent, the TCM may shift harshly, delay or prevent shifts, store limp-mode or default to a basic shift strategy, and the speedometer may read incorrectly. In a Subaru Forester with a CVT, this can impact drivability and safety during acceleration, towing, or hill climbing. Driveability may be reduced until the fault is resolved.

No recalls found in NHTSA database

COMMON CAUSES ON SUBARU FORESTER

• Faulty Output Shaft Speed (OSS) sensor or its connector/wiring (frayed/shorted wires, corroded pins, damaged sensor body).
• Damaged or contaminated OSS sensor harness or poor connector connection at the transmission.
• Wiring harness chafing or pinched conductors near the transmission tubing or bracketry.
• Poor or corroded ground or power supply to the OSS circuit or TCM/ECU.
• Faulty Transmission Control Module (TCM) or a software/learn/adaptation issue requiring a reflash or reset.
• Transmission fluid concerns indirectly affecting sensor readings (low level, degraded CVT fluid, contamination), though the sensor circuit fault is the primary focus.
• Intermittent sensor behavior due to temperature, vibration, or environmental exposure (water intrusion at the connector).
• Sensor or harness installation issues from prior repairs or aftermarket modifications.

SYMPTOMS

• Check Engine Light (CEL) or transmission warning light illuminated.
• Erratic or hard shifting, or delays when shifting through gears or into higher gears.
• Speedometer readings that jump, drop, or drop to 0 when accelerating.
• Cruise control disengagement or erratic cruise control performance.
• Transmission goes into limp mode or limited drive conditions to protect the drivetrain.
• In some cases, the transmission may seem to “hunt” between gears or exhibit poor acceleration response.

DIAGNOSTIC STEPS
Note: Begin with acquiring a proper scan with a capable OBD-II scanner that can read live data and freeze-frame data. If you don’t have access to a professional-grade tool, a reputable auto parts store may help.

Verify the DTC and collect data

• Confirm P0720 is current and check for any related DTCs (P0715 “Input/Turbine Speed Sensor,” P0721 “Output Shaft Speed Sensor Performance,” P0722 “Output Shaft Speed Sensor Range/Performance,” P0730 (Incorrect gear ratio), or P0700 (Transmission Control System Malfunction)).
• Review freeze-frame data and live OSS reading vs vehicle speed. Note whether the OSS reading matches actual road speed and whether it is stable or erratic.

Visual inspection

• Inspect the OSS sensor and its wiring harness at the transmission for signs of damage, wear, oil/water intrusion, or loose/dirty connectors.
• Check for bent pins, corrosion, or ground/earth issues in the connector.
• Look for signs of previous repairs or aftermarket modifications near the sensor harness.

Electrical checks

• With the ignition on (engine off), check that the OSS sensor connector has clean, dry contacts and proper seating in the socket.
• Check continuity of the OSS circuit from the sensor to the ECU/TCM harness connector. Look for shorts to ground or to 12V, and ensure there is a solid ground path.
• Measure sensor resistance (if applicable for your specific OSS sensor). Compare to Subaru service data/specs. If the resistance is out of spec or wildly inconsistent, replace the sensor.
• Check sensor power supply and ground under load (engine running). Look for voltage drop or unstable reference voltage.

Sensor testing and signal validation

• If available, use an oscilloscope to view the OSS signal waveform while the vehicle is being driven or while the speed changes. Steady, clean transitions indicate a healthy sensor; irregular or missing pulses indicate a sensor or wiring fault.
• If you only have a scan tool, watch OSS live data while moving: OSS should rise/fall in step with vehicle speed. If OSS is erratic or fixed while speed changes elsewhere, suspect sensor or wiring.

Basic drivetrain health check

• Check CVT fluid level and condition (level hot, correct type). Contaminated or degraded fluid can affect transmission performance, and in extreme cases, sensor readings may be impacted indirectly via transmission behavior.
• Look for transmission fluid leaks near the sensor area that could affect electrical connections.
• If other transmission-related codes are present (e.g., shift, torque converter, or sensor-related codes), address those in parallel.

If the sensor and wiring pass inspection

• Perform a controlled road test to re-check live data against actual vehicle speed. Clear DTCs and drive in a variety of conditions (idle, light throttle, steady highway speeds, and load situations) to see if the code reoccurs.
• If P0720 returns and the OSS signal remains inconsistent, consider the TCM/ECU as the fault source and plan for reflash, reprogramming, or replacement as per Subaru service literature.

When to replace or escalate

• Replace OSS sensor if the sensor is physically damaged, out of spec, or if resistance/voltage checks fail consistently and other causes have been ruled out.
• Escalate to a professional if the issue persists after OSS replacement, if wiring harness repair is extensive, or if a TCM/ECU fault is suspected (reflashes or module replacement may require dealer tools and software).

RELATED CODES

• P0715 — Input/Turbine Speed Sensor Circuit Malfunction (common companion code when transmission speed sensor circuits exhibit faults)
• P0721 — Output Shaft Speed Sensor Performance
• P0722 — Output Shaft Speed Sensor Range/Performance
• P0730 — Incorrect Gear Ratio
• P0700 — Transmission Control System Malfunction
  Note: Related codes can vary by year and equipment level. Use a full code scan to identify all codes present and address them in a structured manner.

REPAIR OPTIONS AND COSTS (2025 PRICES)
Prices are approximate ranges for common scenarios on 2019-2024 Subaru Forester. Actual prices vary by region, dealer vs independent shop, and current parts availability.

• OSS sensor replacement (parts only): $20–$120

• OSS sensor replacement (labor): 0.3–1.0 hours at typical shop rates ($75–$120/hr)

• Estimated total for OSS sensor replacement: $60–$240

• Wiring harness repair or connector replacement (if harness is damaged or connector corroded)

  • Parts: may be negligible (multi-pin connectors) to $50
  • Labor: 0.5–2.0 hours
  • Estimated total: $75–$350

• TCM/ECU software reflash or programming ( dealer or authorized shop )

  • Reflash/updates: often free if part of a service bulletin (may cost $0–$150 at independent shops)
  • If hardware replacement (new TCM/ECU): $600–$1500+ for parts, plus 1–4 hours of labor
  • Estimated total (with reflash): $100–$400
  • Estimated total (new TCM/ECU): $1000–$2500+

• CVT fluid change/service (if recommended by diagnosis)

  • Parts: CVT fluid and filter (if applicable): $20–$60
  • Labor: 1.0–2.5 hours
  • Estimated total: $150–$350
  • Note: Some Forester CVT fluid services may be more or less expensive depending on the service interval and dealership requirements.

• Transmission replacement (CVT) or major work (rare for P0720 as a sole fault)

  • Parts: CVT unit or rebuilt unit: $1700–$4500 (brand/model dependent)
  • Labor: 6–12 hours
  • Estimated total: $4000–$8000+
  • This is a last-resort scenario if the transmission itself is failing or if sensor-related issues have caused major internal damage.

• Total cost ranges

  • Lowest-cost path (sensor/connector fix only): roughly $60–$300
  • Moderate path (sensor + possible fluid service or minor wiring repair): roughly $150–$700
  • Higher-end path (TCM/ECU work or CVT service): $1000–$5000+, depending on scope

DIY vs PROFESSIONAL

• DIY:
  • Pros: Lower labor cost, educational; ability to test sensors, harness, and connectors yourself; can often identify obvious wiring problems.
  • Cons: Accessing the OSS sensor on a Subaru CVT can require lifting the vehicle or removing panels; electrical testing requires multimeter and possibly an oscilloscope or advanced scan tool; risk of misdiagnosis, incorrect reflash, or accidental damage.
  • What you can do yourself: visually inspect the OSS sensor and wiring, clean connectors with electrical contact cleaner, check for obvious damage, compare live OSS data with vehicle speed, and perform basic resistance checks if you have a service manual spec.
• Professional:
  • Pros: Access to Subaru service data, wiring diagrams, and software tools; precise diagnostics using labs scopes and specialized equipment; warranty coverage for parts and service.
  • Cons: Higher upfront cost; potential wait times for appointments.
• Bottom line: If you are comfortable with electrical diagnostics, have the right tools, and the OSS sensor/wiring shows clear issues, a DIY replacement may be viable. If the diagnosis remains uncertain, or if the CVT and TCM are involved, a professional diagnosis is preferred.

PREVENTION

• Use the correct transmission fluid: Use Subaru-approved CVT fluid and follow the official service intervals. Avoid mixing fluids or using generic alternatives that aren’t approved for the CVT.
• Keep the transmission breathing clean: Regular inspections for leaks and timely fluid changes help maintain sensor reliability and overall transmission health.
• Protect electrical connections: Inspect and clean the OSS sensor connector periodically; ensure seals and grommets are intact to prevent water ingress and corrosion.
• Treat water exposure carefully: After washing the engine bay or driving through heavy rain, inspect for moisture in the OSS connector area and dry as needed.
• Address leaks promptly: Oil or transmission fluid leaks near the sensor can cause electrical issues; repair leaks quickly.
• Avoid aggressive driving patterns that put undue stress on the CVT during cautious learning or break-in periods of new components; gradual driving helps sensors and control modules adapt.
• Keep software up to date: If your Subaru dealer has updates for the ECM/TCM, consider applying them per manufacturer recommendations.

ADDITIONAL TIPS

• If you replace the OSS sensor, consider clearing the transmission adaptation data (per service manual) and performing a proper relearn/relearn drive cycle as required by Subaru to restore smooth shifting.
• If the code reappears after sensor replacement and wiring checks, prepare for a more in-depth diagnosis of the TCM/ECU and possible software updates.

SUMMARY

• P0720 on a 2019-2024 Subaru Forester points to an OSS sensor circuit fault, most often caused by a failing sensor, damaged wiring, or a poor connector. It can affect shifting and speedometer accuracy and may trigger limp mode in some situations.
• Begin with a thorough visual inspection and electrical testing of the OSS sensor and its harness, verify with live data against vehicle speed, and address any wiring or connector faults. If the sensor and wiring appear sound, consider TCM/ECU software or hardware issues.
• Costs can range from a modest sensor replacement to more extensive transmission work. The most cost-effective approach is to confirm sensor integrity and harness condition before moving to major components.
• There are no recalls found in the NHTSA database for this exact make/model/year/issue combination in the provided data, and no recalls are cited here. Use the information above as a diagnostic guide in light of that limitation and consult a Subaru specialist for model-specific service data.