P0137
O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)P0137 is a generic OBD-II powertrain diagnostic trouble code: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2). It is logged by the engine control unit when the o2/lambda monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0137 means
P0137 is set when the ECM measures the downstream oxygen sensor (Bank 1 Sensor 2) signal voltage below approximately 0.1 V for longer than the allowed monitoring window. The post-catalyst sensor operates differently from the upstream sensor: in a healthy system with an efficient catalytic converter, most of the oxygen variations in the exhaust have been smoothed out, and the sensor should sit at a relatively stable 0.5–0.8 V. A persistently low voltage indicates that the sensor is seeing a very lean—or oxygen-rich—exhaust stream, or that the sensor element or its signal circuit has failed in the low direction.
The most common cause is a failed sensor element that has degraded to the point where it can no longer generate the correct electrochemical voltage. Second in frequency are wiring faults: the signal wire from the downstream sensor runs a long path to the ECM, often along the floorpan and near suspension and exhaust components, making it vulnerable to chafing and corrosion over time. An exhaust leak downstream of the catalyst is another cause, as fresh air introduced near the sensor tip creates an artificially lean atmosphere at the element.
While the downstream sensor does not directly drive fuelling corrections on most modern ECMs, a failed P0137 state will prevent the catalyst efficiency monitor (P0420) from running correctly, and the MIL will illuminate. Cross-referencing with P0420 and live fuel-trim data helps distinguish a sensor failure from a genuine lean exhaust condition reaching the downstream sensor.
Common causes
Most-frequently reported root causes when P0137 is logged.
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1
Degraded or failed downstream HO2S sensor element (most common; sensor biased or stuck low)
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2
Signal wire shorted to chassis ground or exhaust shielding
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3
Corroded or water-ingressed sensor connector near the underfloor routing
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4
Exhaust leak downstream of the catalyst introducing fresh air near the sensor bung
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5
Open or high-resistance sensor ground circuit
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6
Sensor heater failure causing element to remain below operating temperature in cold conditions
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7
Genuine very lean exhaust composition reaching the post-cat sensor (vacuum leak, weak injectors)
Symptoms drivers notice
How to diagnose P0137
A typical diagnostic flow when this code is present.
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1
Read all stored DTCs and freeze-frame data; concurrent P0171 (system lean Bank 1) alongside P0137 suggests genuine lean exhaust is reaching the downstream sensor, requiring upstream diagnosis first
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2
Inspect the underfloor sensor harness from the bung to the cabin firewall connector; look for chafing against heat shields, corroded splice points, or water damage in low-lying connector housings
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3
Check for exhaust leaks in the section of pipe between the catalytic converter outlet and the downstream sensor bung with the engine running
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4
With engine fully warm, monitor live HO2S B1S2 voltage on a scan tool; a flat signal at 0.00–0.08 V that does not respond when the upstream sensor oscillates confirms the sensor or circuit is at fault
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5
Disconnect the sensor connector and measure: (a) heater circuit resistance at sensor pins (typically 4–30 Ω); (b) signal wire to chassis ground resistance (must be >10 kΩ—lower indicates a short to ground)
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6
Verify ECM reference voltage is present on the signal circuit with the sensor disconnected (approximately 0.45 V bias); absence of bias indicates an open or ECM-side fault
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7
Replace the downstream O2 sensor if all circuit tests pass; clear codes, run a full drive cycle including highway steady-state to allow the catalyst monitor to complete
Related powertrain codes
- P0040 — Upstream Oxygen Sensors Swapped From Bank To Bank
- P0041 — Downstream Oxygen Sensors Swapped From Bank To Bank
- P0130 — O2 Sensor Circuit Malfunction (Bank 1 Sensor 1)
- P0131 — O2 Sensor Circuit Low Voltage (Bank 1 Sensor I)
- P0132 — O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)
- P0133 — O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
Frequently asked questions
Will P0137 affect my fuel consumption?
Usually not significantly. On most modern vehicles, the downstream sensor is used primarily for catalyst monitoring and trim fine-tuning, not primary fuel control. The upstream sensor controls the bulk of closed-loop corrections, so a downstream fault rarely causes a measurable fuel economy change.
How long does the downstream O2 sensor typically last?
OEM sensors generally last 100,000–160,000 km under normal conditions. The downstream unit actually tends to last slightly longer than the upstream sensor because it runs at lower temperatures, but its wiring is more exposed to underfloor corrosion and mechanical damage.
Can I clear P0137 and just monitor it?
Clearing the code will turn off the MIL temporarily, but it will return within one or two drive cycles if the fault is still present. More importantly, with P0137 active the catalyst efficiency monitor cannot run, so a developing P0420 condition would go undetected. It is better to diagnose and repair the fault.
Is the downstream sensor on Bank 1 always on the driver's side?
Bank 1 is the cylinder bank that contains cylinder #1. On inline-4 and inline-6 engines there is only one bank, so Bank 1 is straightforward. On V6 and V8 engines, Bank 1 is typically the driver's side in North American vehicles, but this varies by manufacturer and engine orientation—always confirm with a vehicle-specific wiring diagram.
Disabling P0137 in software
RaceTune can permanently disable P0137 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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