P0137 means the Forester Bank 1 downstream oxygen-sensor signal stayed low. Check exhaust leakage, upstream mixture and misfire before load-testing rear-sensor signal, heater and harness. Subaru bulletin 09-115-24 covers identified 2019–2023 Forester applications with a rear-sensor design change. A separate bulletin 11-169-16R includes P0137 in a narrow 2014–2017 Forester turbo ECM branch and instructs technicians to eliminate improper fuel first. Neither bulletin applies from code wording alone.
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 |
Two Subaru bulletins describe different branches
The 2024 sensor bulletin concerns identified rear O2 sensor production on listed later models. The turbo ECM bulletin concerns particular 2014–2017 FA20DIT calibrations and requires checking fuel quality first.
Do not combine them into a universal Forester repair. VIN, engine, ECM part number and sensor identification determine whether either branch applies.
Prove whether low voltage is electrical or real
Graph rear voltage, heater status, upstream A/F response and fuel correction after warm-up. A circuit pinned low with failed loaded tests differs from a responsive sensor seeing oxygen from a lean exhaust or leak.
Inspect connectors and harness near heat shields and underbody points, then inspect the exhaust cold for soot or cracks ahead of the sensor.
Clear upstream faults before catalyst conclusions
Resolve P0171 or P0300 because mixture and misfire directly affect downstream oxygen. Subaru's P0420 diagnostic bulletin treats P0137 as a prerequisite code.
Verify the repair through the hot monitor with plausible sensor response and no pending P0137. A software update cannot seal an exhaust leak or repair a damaged circuit.
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
Forester diagnostics span SF through SK generations with EJ-series SOHC and turbo engines, FB25 and FA20 direct-injection turbo applications. AVLS and AVCS are different systems, bank orientation matters, and catalyst, EVAP and oxygen-sensor hardware changes by year and engine. Confirm VIN, model year, engine code, turbo status, emissions label and current ECM calibration before using any test or bulletin.
- 09-115-24 requires sensor identification on listed 2019–2023 Foresters.
- 11-169-16R is limited to specified 2014–2017 turbo ECM applications.
- Naturally aspirated and turbo exhaust routing differs.
- P0171, P0300 and P0420 companions change diagnostic priority.
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 11-169-16R — 2014–2017 Forester turbo ECMNarrow FA20 turbo reprogramming branch that includes P0137; fuel quality and exact ECM applicability must be checked first↗03Subaru 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↗04Subaru Technical Information System (STIS)Current VIN-, engine- and model-year-specific service-manual diagnostic procedures↗05Subaru owner manuals and vehicle resourcesOfficial model-year operating, warning, fuel and maintenance information↗