P0137 The downstream oxygen sensor on bank 1 is reporting a voltage that stays too low

O2 sensor circuit low voltage, bank 1 sensor 2

Monitor

The rear sensor mainly monitors the catalytic converter rather than controlling fuel, so the car normally drives the same; book a diagnosis rather than stopping.

It gets more serious if:

  • Stop: Stop if the check engine light starts flashing: that means an active misfire hot enough to damage the catalytic converter. Pull over where nothing flammable is under the car and let it cool.

Source: how we rate severity

P0137 in short

P0137 means the oxygen sensor after the catalytic converter on bank 1 is reading a voltage that stays too low. Common causes are an exhaust leak between the sensors, a wiring fault or a worn sensor. The car usually drives normally, but fix it before judging the converter.

The downstream sensor (bank 1 sensor 2) sits after the catalytic converter and checks that the converter is working. Its voltage normally rises when the exhaust is rich and falls when it is lean, and a healthy converter keeps it fairly steady.

P0137 sets when that voltage stays below the range a working sensor should produce. On the 2008 Saab 9-3, for example, it sets when the input stays below about 0.06 volts under the test conditions. A broken or grounded signal wire, outside air leaking into the exhaust between the sensors, or a worn sensor can all do this.

Because the computer uses this sensor to grade the catalytic converter, a faulty reading can lead to a catalyst code and an unnecessary converter replacement. Diagnose other codes first, then the wiring, then exhaust leaks, and replace the sensor last.

Most likely causes

These are general causes for this code across vehicles. None is specific to one make or model.

  1. Exhaust leak between the front and rear oxygen sensors

    Common general cause
    Why it ranks here
    Outside air drawn in through a crack or loose joint makes the rear sensor read lean, which is a low voltage. Saab's procedure checks for exactly this before replacing the sensor.
    What would change this
    A crack, hole, loose joint or soot track between the two sensors confirms it.

  2. Signal wire open or shorted to ground, or a poor connection

    Common general cause
    Why it ranks here
    A broken or grounded signal wire reads as a very low voltage. Saab's service information treats this code as an electrical fault in the leads or the sensor.
    What would change this
    A wire that fails a continuity check, or a reading that changes when the harness is wiggled, confirms it.

  3. Failed or slow rear oxygen sensor

    Common general cause
    Why it ranks here
    Sensors wear out with mileage and stop producing a normal signal. Saab's procedure replaces the rear sensor once wiring and exhaust leaks are ruled out.
    What would change this
    A sensor that stays low even during hard acceleration, with good wiring and no leaks, confirms it.
    How to test the oxygen (o2) sensor

  4. Engine genuinely running lean

    Possible — needs testing
    Why it ranks here
    If the whole engine is lean, the rear sensor reads low too. Saab's procedure has other codes diagnosed first for this reason.
    What would change this
    Lean codes (P0171, P0174) or high positive fuel trims raise this; normal trims lower it.

  5. Reference voltage fault at the sensor connector

    Possible — needs testing
    Why it ranks here
    Saab's procedure checks the reference voltage at the connector before condemning the sensor.
    What would change this
    Incorrect reference voltage with sound wiring raises this.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Read all codes; diagnose fuel-trim, misfire and other oxygen sensor codes first

    Low riskTool: OBD-II scan tool

  2. With the engine fully warm, read the rear (bank 1 sensor 2) voltage at idle and while holding a higher engine speed or accelerating briefly; it should rise well above its low limit when the mixture goes rich

    Use cautionTool: scan tool with live data

    Warning:Keep hands, hair and loose clothing clear of the belts and fan while the engine runs, and never run the engine in a closed garage.

    • IfThe voltage never rises, even under acceleration

      It meansThe signal is stuck low; check the wiring.

      Go to step 3

    • IfThe voltage rises normally now

      It meansThe fault is intermittent or caused by a leak; check the exhaust.

      Go to step 4

  3. With the exhaust cold, inspect the rear sensor wiring and connector, then check reference voltage and signal-wire continuity

    Use cautionTool: digital multimeter; wiring diagram

    Warning:Let the exhaust cool fully before touching the sensor or its wiring. Back-probe connectors gently, never pierce insulation, and follow the wiring diagram for your engine.

  4. With the exhaust cold, check for cracks, holes or loose joints between the front and rear sensors

    Use cautionTool: visual inspection; smoke machine (shop)

    Warning:Exhaust parts stay hot enough to burn skin long after the engine stops. Let the car cool for at least an hour, and never get under a car held up only by a jack.

Symptoms

  • Check engine light with no change in how the car drives
  • Catalyst efficiency code (P0420) stored as well
  • Failed or refused emissions inspection

How to fix

  1. With the exhaust fully cold, inspect the rear oxygen sensor's wiring for melted insulation or chafing against the exhaust, and check its connector is latched and free of corrosion.

  2. Look along the exhaust between the front and rear sensors for holes, cracks or loose joints.

Parts and cost context

  • Oxygen (O2) sensor:Replace the rear sensor only after wiring, exhaust leaks and other codes are ruled out, and use the correct part for that position.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Subaru

  • 2015 · Legacy

    Supported by vehicle data

    On 2015 Legacy and Outback, Subaru released ECM reprogramming for P0137; check for it before replacing the rear sensor.

  • 11-160-15Status unverified

    ECM reprogramming for P0137

    2015legacy, outback · issued · NHTSA 10093110

    Subaru released ECM reprogramming files to address P0137 on listed 2015 Legacy and Outback models.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • P0138The downstream oxygen sensor on bank 1 is reporting a voltage that stays too high

    Same system Common general cause

    P0138 is the opposite fault on the same rear sensor circuit and follows the same diagnostic path: other codes, wiring, exhaust leaks, then the sensor.

  • P0420Catalytic converter efficiency below the computer's limit (Bank 1)

    Primary / secondary Common general cause

    Likely primary: P0137. The catalyst test relies on this sensor. Fix P0137 first; Saab's procedure has other codes diagnosed before P0420, and a faulty rear sensor can fail a good converter.

  • P0171Engine computer has reached its limit adding fuel on bank 1: the mixture is lean

    Primary / secondary Possible — needs testing

    Likely primary: P0171. A genuinely lean engine pushes the rear sensor low. If P0171 is stored, fix the mixture first.

All Oxygen sensors codes

P0137 questions

Can I drive with P0137?

Usually yes; the rear sensor doesn't control fuelling on most cars, so the engine runs the same. Book a diagnosis, because the fault stops the converter from being checked and can lead to a failed inspection.

Which oxygen sensor is bank 1 sensor 1, and which is sensor 2?

Bank 1 is the side of the engine with cylinder 1; an inline engine has only bank 1. Sensor 1 is the upstream sensor before the catalytic converter; sensor 2 is the downstream sensor after it. Check a diagram for your engine before buying a part.

Will replacing the sensor fix P0137?

Only if the sensor is the fault. Wiring damage near the exhaust and exhaust leaks between the sensors set the same code, so check those first.

Can an oxygen sensor fault damage my catalytic converter?

It can, over time. The EPA notes that fixing a failing oxygen sensor promptly can save you from replacing the much more expensive catalytic converter later.

Will my car pass an emissions inspection with this code?

Not while the check engine light is on; a lit light with stored codes fails the federal OBD inspection. After the repair the light normally goes out after three good drives, and the oxygen sensor and catalyst monitors must complete before a retest.

Sources and review

  1. Bosch Oxygen Sensors consumer brochure — Robert Bosch LLC ,
  2. Saab 9-2X 2.5i (2005) WIS DTC P0137: O2 sensor circuit low voltage (bank 1 sensor 2) — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  3. Final Regulation Order: 13 CCR §1968.2, Malfunction and Diagnostic System Requirements (OBD II) for 2004 and later passenger cars, light-duty trucks and medium-duty vehicles — California Air Resources Board
  4. 2026 Honda Pilot Owner's Manual: Indicator Coming On/Blinking (Malfunction Indicator Lamp) — American Honda Motor Co.
  5. P0420 Code: The Guide to Diagnosis and Fast Fixes — Identifix ,
  6. Saab 9-3 (2008) WIS DTC P0137 00 (ME9): O2S 2 circuit low voltage — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  7. Bosch Oxygen Sensors trade brochure — Robert Bosch LLC ,
  8. P0171 Code: Causes, Symptoms, and How to Fix It — Identifix ,
  9. Saab 9-2X 2.5i (2005) WIS DTC P0138: O2 sensor circuit high voltage (bank 1 sensor 2) — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  10. Saab 9-2X 2.5i (2005) WIS DTC P0420: Catalyst system efficiency below threshold (bank 1) — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  11. Subaru of America bulletin 11-160-15: ECM reprogramming for P0137 — Subaru of America (filed with NHTSA) ,
  12. A/F Sensors — DENSO Auto Parts
  13. 40 CFR §85.2222 Onboard diagnostic test procedures — U.S. Environmental Protection Agency (Electronic Code of Federal Regulations)
  14. OBD: Frequently Asked Questions (EPA420-F-02-014) — U.S. Environmental Protection Agency, Office of Transportation and Air Quality ,
Last reviewed
Written by
Saurabh K.
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