P0137 means the Bank 1 downstream oxygen-sensor signal stayed lower than expected. Confirm the rear sensor rather than the upstream A/F sensor, inspect the exhaust for air entry, graph fuel correction and both sensor signals, and load-test the signal, ground, heater and harness. Subaru bulletin 09-115-24 covers identified late-model Outback applications and requires a sensor-identification procedure; it does not authorize replacement on every year. Earlier calibration listings are also application-specific.
Match the symptom
Select the closest observation. The result changes the first test; it does not name a part to replace.
Graph rear sensor voltage and heater status before parts.
What can set P0137—and how to separate it
The order below is evidence-driven, not a universal probability ranking. Vehicle year, repair history, companion codes and the operating condition in freeze frame change where diagnosis should begin.
| Possible cause | Evidence that supports it | First useful test |
|---|---|---|
| Rear oxygen sensor internal or circuit fault | Signal/electrical test fails under applicable conditions | Voltage, heater and loaded-circuit test |
| Exhaust leak ahead of Bank 1 Sensor 2 | Cold tick, soot or smoke/pressure evidence | Exhaust leak inspection |
| True lean exhaust from fuel/air fault | Upstream trims and companion codes agree | Fuel-control diagnosis |
| Connector contamination, heat or harness damage | Visual or wiggle/load evidence | Connector and routing test |
| Applicable sensor-design or ECM-calibration branch | Exact model year, engine and identification match | Subaru bulletin applicability |
Low voltage can be real oxygen or an electrical fault
Graph Bank 1 Sensor 2 after warm-up while recording upstream correction. A signal pinned near zero with a failed circuit test differs from a responsive sensor reporting a real lean exhaust stream.
Load-test heater supply, signal return, ground and terminal tension. Exhaust heat, water and prior repair can damage routing even when static continuity looks acceptable.
Inspect the exhaust before replacing the sensor
Check joints, welds, flex sections and the sensor bung cold for soot or a tick. Outside air entering ahead of the sensor can hold the reported voltage low.
Resolve P0171 or active misfire because those conditions change exhaust oxygen. If P0420 is present, Subaru's catalyst bulletin lists rear-sensor and mixture faults among prerequisites.
Apply the production change by identification, not keyword
Bulletin 09-115-24 describes an optimized rear-sensor design and a required identification process for listed applications. Model name and P0137 alone do not establish that the vehicle has the superseded sensor.
After the proven circuit, leak, mixture or sensor fault is repaired, complete the hot monitor and confirm plausible rear-sensor response with no pending P0137.
Save these scan-data clues
Freeze frame is a snapshot of conditions when the PCM matured the fault. It can distinguish an idle-only problem from a load, cold-start or monitor-completion problem. Clearing first throws that context away.
- Exact VIN, platform, model year and engine sales code
- All confirmed, pending and permanent companion codes
- RPM, load, coolant temperature and vehicle speed
- Relevant commanded and actual scan data
- Recent service, repairs and symptom conditions
- Manufacturer bulletin and current software/VIN status
A practical diagnostic workflow
- 01Confirm Bank 1 Sensor 2 and engine
Do not confuse the rear O2 sensor with the upstream A/F sensor.
- 02Save front/rear data and companions
Record trims, temperature and heater status.
- 03Inspect exhaust and harness cold
Look for soot, cracks, heat damage and connector strain.
- 04Load-test sensor and heater circuits
A low signal can be wiring or sensor related.
- 05Prove upstream mixture control
Resolve lean, rich or misfire evidence first.
- 06Check exact Subaru bulletin identification
Production changes and calibration files are not universal repairs.
- 07Verify hot monitor operation
Confirm plausible response and no pending P0137.
Do not replace the most commonly named component from P0137 alone. Confirm the exact engine, preserve evidence and complete the manufacturer test sequence before ordering parts.
Why the model year changes the test
Outback diagnostics span Legacy-based early models through modern crossovers with EJ25, EZ30/EZ36, FB25 and FA24 turbo engines. Valve-lift hardware, AVCS strategy, fuel injection, bank layout, rear oxygen sensors, catalysts and EVAP leak detection changed repeatedly. Confirm VIN, model year, engine code, aspiration, emissions label and current ECM calibration before applying a code definition, bank location or bulletin.
- 09-115-24 includes 2020–2024 Outback 2.5L applications with identification boundaries.
- Six-cylinder and turbo exhaust packaging may differ.
- P0171, P0300 or P0420 companions must be interpreted before a sensor verdict.
- Rear O2 and upstream A/F sensors are not interchangeable.
P0137 FAQ
Is Bank 1 Sensor 2 the downstream sensor?+
Yes, on these Subaru applications it is the rear oxygen sensor after the Bank 1 catalyst; confirm the exact exhaust layout.
Does P0137 prove the sensor is bad?+
No. Wiring, exhaust leakage and a real lean condition can also hold the signal low.
Can I drive?+
Usually briefly with a steady lamp and normal running.
Can an exhaust leak cause it?+
Yes, if outside oxygen reaches the exhaust ahead of the sensor.
Is the upstream sensor the same part?+
No. Subaru commonly uses an upstream air/fuel sensor with different operation.
Does Subaru have a sensor update?+
Yes for identified applications, but the sensor identification and model-year boundaries must match.
Should P0420 be diagnosed first?+
Use the complete code set; sensor circuit and mixture faults are prerequisites before a catalyst verdict.
What confirms repair?+
A valid rear-sensor signal and completed monitor with no pending P0137.
Ford diagnostic references
These documents explain Ford monitor operation. Exact pinpoint tests, connector views and specifications must be selected for the truck's VIN and model year in current service information.
01Subaru Bulletin 09-115-24 — rear oxygen-sensor identificationCovers identified late-model Outback and Forester applications with P0137/P0138/P0140/P0141; requires the published sensor-identification procedure↗02Subaru Bulletin 09-110-23R — P0420 diagnostic procedureCovers listed Outback and Forester years and requires companion-code, mixture, misfire, sensor and exhaust checks before catalyst judgment↗03Subaru Technical Information System (STIS)Current VIN-, engine- and model-year-specific service-manual diagnostic procedures↗04Subaru owner manuals and vehicle resourcesOfficial model-year operating, warning, fuel and maintenance information↗