P0137 means the WRX Bank 1 downstream oxygen-sensor signal stayed low. Graph rear voltage with upstream A/F correction, inspect the turbo exhaust for air leakage, and load-test signal, ground, heater and heat-exposed wiring before replacing the sensor. Subaru bulletin 09-115-24 covers other named models but does not list WRX, so its revised-sensor branch must not be borrowed for this page. The WRX uses its VIN-correct STIS procedure; P0171, P0300 and P0420 companions take priority.
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 |
A low signal can be real oxygen or circuit loss
Graph rear voltage after full warm-up alongside upstream correction. A responsive sensor tracking a true lean condition differs from a circuit pinned low regardless of operating change.
Load-test heater supply, signal return and ground while inspecting heat shields, connectors and areas altered by exhaust work. Static continuity may pass through a weak hot connection.
Inspect the turbo exhaust cold
Check cylinder-head, turbo, downpipe/front-pipe, flange and sensor-bung joints for soot or a cold tick. Outside air ahead of the rear sensor can hold its signal low.
Resolve P0171 and active misfire first. Subaru's catalyst bulletin screens P0137 before it evaluates P0420, preventing sensor or mixture faults from becoming converter diagnoses.
Do not misapply the 2024 sensor bulletin
Bulletin 09-115-24 lists Legacy/Outback, Impreza/Crosstrek, Forester and BRZ applications, not WRX. Similar code wording does not expand its production boundaries.
Use current WRX STIS and verify plausible hot rear-sensor response with no pending P0137. A universal sensor order would ignore engine and exhaust changes.
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
WRX diagnostics span EJ20/EJ255 turbo models, FA20DIT direct injection and the newer FA24DIT, while STI variants retain different EJ257 hardware in many years. Intake/exhaust AVCS coverage, turbo and EGR plumbing, cylinder banks, fuel systems, catalysts and ECM monitor thresholds are not interchangeable. Confirm VIN, model year, WRX versus STI, engine code, transmission, emissions label and ECM calibration before using any test or bulletin.
- 09-115-24 does not list WRX among its applicable models.
- Turbo exhaust joints and aftermarket downpipes change leak and wiring context.
- Rear O2 and upstream A/F sensors have different functions.
- P0420 bulletin 09-110-23R treats P0137 as a prerequisite on covered WRXs.
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 design changeLists Outback, Impreza/Crosstrek, Forester and BRZ applications but not WRX; useful chiefly to prevent misapplying that sensor branch↗02Subaru Bulletin 09-110-23R — P0420 diagnostic procedureCovers 2018–2023 WRX and requires misfire, mixture, sensor, oil-smoke and exhaust checks plus model-specific SSM verification↗03Subaru Technical Information System (STIS)Current VIN-, engine- and model-year-specific diagnostic procedures↗04Subaru owner manuals and vehicle resourcesOfficial model-year operating, warning, fuel and maintenance information↗