P0137 — Post-Cat O2 Sensor Low Voltage (Bank 1 Downstream)

What does P0137 mean?

P0137 is set when the Bank 1 downstream oxygen sensor reports a voltage below the calibrated minimum — typically under about 0.1 V — for an extended period. Bank 1 is the cylinder bank containing cylinder number 1. Sensor 2 is the sensor AFTER the catalytic converter, not the one threaded into the exhaust manifold. A healthy downstream sensor on a warm engine holds a relatively steady 0.6–0.8 V because the catalyst smooths out the rich / lean swings coming from the engine, so a reading pinned low means an open or shorted-to-ground signal circuit, a failed sensor, or fresh air getting into the exhaust ahead of the sensor.

What are the symptoms of P0137?

• Check Engine Light is illuminated
• Catalyst readiness monitor will not complete
• Often no drivability symptoms at all
• Possible slight reduction in fuel economy
• Vehicle will fail emissions / smog testing
• Sometimes accompanied by P0171 if a genuine lean condition is present

What causes P0137?

Repair costs are typical US ranges and vary by make, model, model year, and labor rate. A diagnostic trouble code is a symptom, not a guaranteed failed part — confirm the root cause before replacing anything.

Is it safe to drive with P0137?

In most cases a vehicle stays drivable for short trips with P0137 active, but you should diagnose and repair it promptly. This is a moderate-severity code — ignoring it can lead to further damage or a failed emissions test. Exact tolerance depends on your specific make and model.

How to diagnose P0137

1. Confirm you are at the right sensor before touching anything

   Bank 1 Sensor 2 is downstream of the catalytic converter on the bank containing cylinder number 1. On an inline engine there is only one bank, so it is simply the sensor after the cat. On a V engine you have to establish which bank holds cylinder 1 first. Replacing the upstream sensor by mistake is the single most common wasted repair on this code.

   Tools: Vehicle-specific service information, Vehicle lift or jack stands

2. Read fuel trims to separate a real lean condition from a sensor fault

   Check Bank 1 long-term fuel trim at idle and at 2500 RPM. Near zero means the engine is fuelling correctly and the sensor or its wiring is lying. Trim at +15 % or higher means the engine really is lean, the sensor is reporting honestly, and your job is to find the vacuum or exhaust leak — not to buy an O2 sensor.

   Tools: Scan tool with live fuel trim PIDs

3. Disconnect the sensor and look for bias voltage

   Unplug the downstream sensor. With the key on, the scan tool PID should show the ECM's bias voltage, usually around 0.4–0.5 V. If the reading stays pinned at zero with the sensor disconnected, the signal wire is shorted to ground somewhere between the connector and the PCM, and a new sensor will not change a thing.

   Tools: Scan tool, Multimeter

4. Inspect the harness and connector under the vehicle

   The downstream sensor and its harness live under the car in road spray and salt. Trace the wiring from the sensor body back to the chassis connector looking for melted insulation where it has touched the exhaust, chafing against the underbody, and rodent damage. Open the connector and check for water and green corrosion.

   Tools: Flashlight, Inspection mirror, Electrical contact cleaner

5. Find exhaust leaks between the catalyst and the sensor

   A pinhole or a leaking flange gasket ahead of the downstream sensor lets fresh air into the exhaust stream, and the sensor correctly reports the oxygen it sees — as a permanently low voltage. Listen along the pipe with the engine cold, or use a smoke machine on the exhaust to make small leaks visible.

   Tools: Mechanic's stethoscope, Smoke machine (optional), Flashlight

How do I fix P0137?

• Replace the Bank 1 downstream O2 sensor with an OEM part
• Repair the shorted-to-ground or open O2 signal wire
• Repair exhaust leaks between the catalyst and the downstream sensor
• Clean or replace the corroded downstream sensor connector
• Repair vacuum leaks if fuel trims confirm a genuine lean condition

P0137 by make and model

Select your exact year, make, and model below to get a P0137 diagnosis tuned to your vehicle's known failure patterns, recalls, and TSBs.

Related diagnostic codes

• P0136 — Post-Cat O2 Sensor Circuit Fault (Bank 1)
• P0138 — Post-Cat O2 Sensor High Voltage (Bank 1 Downstream)
• P0140 — Post-Cat O2 Sensor No Activity (Bank 1 Downstream)
• P0141 — Post-Cat O2 Heater Circuit (Bank 1 Downstream)
• P0157 — Post-Cat O2 Sensor Low Voltage (Bank 2 Downstream)
• P0171 — Fuel System Too Lean (Bank 1)

Common symptoms of P0137

P0137 often shows up as one of these symptoms. Start from what your car is actually doing:

• Poor fuel economy
• Failed emissions / smog test