Buy Subaru Legacy Oxygen Sensors

Most modern Legacy models have an upstream sensor (Sensor 1) mounted in the exhaust manifold or front pipe before the catalytic converter, and a downstream sensor (Sensor 2) mounted after the converter. ÒBank 1Ó means the side of the engine with cylinder #1 (on single-bank engines, that's the only bank). This bank/sensor naming is standard across OBD-II cars. (OBD-Codes.com)

• The upstream unit on many Subarus is a wideband A/F sensor. It measures the mixture very precisely so the engine can adjust fuel quickly.
• The downstream unit is typically a narrowband O₂ sensor that mainly watches catalyst efficiency.
  Subaru diagnostic literature lists A/F Sensor #1 data and Rear O₂ Sensor data separately-which shows the different roles. (subaruport.ru)

Subaru service and warranty materials list both Air/Fuel Ratio Sensor and Oxygen Sensor as emission-related parts on late-model vehicles (Legacy included). That matches what you'll see on a scan tool: an A/F sensor upstream and an O₂ sensor downstream on many 4-cyl Legacys. For official service procedures and diagrams, Subaru hosts them in STIS (Subaru Technical Information System).

Common symptoms of a failing or ÒlazyÓ oxygen sensor or A/F sensor: Check Engine Light, poor MPG, rough running, and failed emissions. A faulty sensor can also lead to catalytic converter damage over time. (Wikipedia)

These OBD-II codes often appear when sensors or their circuits act up:

• P0130 - O₂ Sensor Circuit Malfunction (Bank 1 Sensor 1)
• P0131 - O₂ Sensor Low Voltage (B1S1)
• P0137 - O₂ Sensor Low Voltage (B1S2)
• P0141 - O₂ Sensor Heater Circuit Malfunction (B1S2)
• P2A00 / P2A01 - O₂ sensor range/performance (upstream/downstream)
• P0420 - Catalyst efficiency below threshold (sensors used to detect this)
  Each code points to a bank and sensor number, which helps you find the right part and test the right circuit. (OBD-Codes.com)

Heat, age, and contamination are the big reasons. Coolant leaks, oil burning, silicone sealers, and some fuel additives can poison a sensor. That makes it slow to respond or unable to read correctly. Manufacturers' tech guides show typical ÒpoisonedÓ sensors and what caused them. (Wikipedia, Bosch Auto Parts, Walker Products)

Most modern heated sensors last close to 100,000 miles. There's usually no scheduled replacement-you replace them when they fail testing or set a code. (Wikipedia)

1. Scan for codes and grab freeze-frame data.
2. On a warm engine in closed loop, upstream sensor data should change quickly with throttle; downstream should stay steadier (if the catalyst is healthy).
3. Look for exhaust leaks or wiring issues before blaming the sensor.
   These checks mirror how technicians judge sensor behavior and readiness. (OBD-Codes.com, subaruport.ru)

Clear codes, then complete a drive cycle so readiness monitors (including O₂ and catalyst) flip to ÒcompleteÓ for inspection. EPA and state guides explain what readiness is and why it matters. (US EPA, epa.ohio.gov, nyvip3.com)

Subaru lists Air/Fuel Ratio Sensor and Oxygen Sensor under emissions warranty parts (short-term coverage varies by state rules; some parts have extended coverage in certain programs). Also, tampering with emissions controls (including O₂ sensors) or using defeat devices is illegal under the Clean Air Act. (US EPA)

Want to go deeper? Try these plain-language primers:

• How O₂ sensors work and fail (encyclopedic overview). (Wikipedia)
• Common O₂/A/F sensor trouble codes and meanings. (OBD-Codes.com)
• Subaru service & warranty documentation (official).
• Readiness monitors explained for inspections. (US EPA)

• Let the exhaust cool fully.
• Use an O₂-sensor socket and penetrating oil.
• Avoid twisting the harness; keep the connector clean and dry.
• Many new sensors arrive with pre-applied anti-seize on the threads-don't add extra, and don't put dielectric grease in the connector. That can block the sensor's needed reference air. (Bosch Auto Parts)

• Names you'll see: oxygen sensor, O₂ sensor, lambda sensor, A/F (air-fuel ratio) sensor.
• Upstream vs. downstream: Upstream (Sensor 1) controls fuel; downstream (Sensor 2) watches the catalyst.
• Best next steps: Scan for codes, confirm with live data, fix leaks/wiring first, then replace only what tests bad. Clear codes and complete a drive cycle before inspection. (OBD-Codes.com, US EPA)

If you want, I can tailor this page to a specific Legacy year/engine and add model-specific locations and test values from Subaru service docs.