A faulty downstream oxygen sensor that has drifted out of specification or has an internal short; Damaged or corroded wiring between the sensor and the PCM, including chafed insulation or broken pins; A poor ground connection for the sensor heater circuit or the sensor body itself; An exhaust leak upstream of the sensor, allowing outside air to enter and cause a false lean reading; A vacuum leak in the intake system that creates a genuinely lean exhaust condition; A failing catalytic converter that is not storing oxygen properly, though this usually sets a different code; Low fuel pressure or a clogged fuel injector causing a lean condition that the downstream sensor detects

DTC codes / P01xx · Fuel and air metering / P0157

P0157O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)

Q: What does code P0157 mean?

O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2). P0157 means the powertrain control module (PCM) has detected a low-voltage signal from the downstream oxygen sensor on bank 2, which is the side of the engine that does not contain cylinder number one. The sensor is reporting a voltage below the expected threshold for the current operating conditions, indicating either a lean exhaust condition, a wiring fault, or a failing sensor.

P0157 means the powertrain control module (PCM) has detected a low-voltage signal from the downstream oxygen sensor on bank 2, which is the side of the engine that does not contain cylinder number one. The sensor is reporting a voltage below the expected threshold for the current operating conditions, indicating either a lean exhaust condition, a wiring fault, or a failing sensor.

SystemPowertrain

Code typeGeneric (all makes)

SeverityLow

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
A slight decrease in fuel economy as the PCM may default to a richer fuel mixture
Occasional rough idle or hesitation during acceleration if the low voltage is caused by a lean condition
A failed emissions test due to the sensor circuit fault
In rare cases, a sulfur or rotten egg smell from the exhaust if the PCM over-fuels in response

Common causes, most likely first

A faulty downstream oxygen sensor that has drifted out of specification or has an internal short
Damaged or corroded wiring between the sensor and the PCM, including chafed insulation or broken pins
A poor ground connection for the sensor heater circuit or the sensor body itself
An exhaust leak upstream of the sensor, allowing outside air to enter and cause a false lean reading
A vacuum leak in the intake system that creates a genuinely lean exhaust condition
A failing catalytic converter that is not storing oxygen properly, though this usually sets a different code
Low fuel pressure or a clogged fuel injector causing a lean condition that the downstream sensor detects

How to diagnose and fix it

Inspect the sensor wiring and connector for damage, corrosion, or loose pins, and repair as needed
Check for exhaust leaks between the engine and the sensor, including gaskets and pipe joints
Use a scan tool to view live data for the bank 2 sensor 2 voltage and compare it to the bank 1 sensor 2 to see if the reading is plausible
Perform a propane enrichment test or smoke test to check for vacuum leaks that could cause a lean condition
Measure the sensor heater resistance and supply voltage to rule out a heater circuit fault
Replace the downstream oxygen sensor if the wiring and exhaust are intact and the sensor response is sluggish or stuck low
Clear the code and verify the repair by completing a drive cycle and confirming the monitor runs without resetting the code

Downstream sensor codes are not always the sensor

A low voltage reading from a downstream oxygen sensor often indicates a lean exhaust condition rather than a failed sensor, so technicians should check for vacuum leaks and exhaust leaks before replacing parts.
The downstream sensor's primary job is to monitor catalytic converter efficiency, so a code P0157 can be set by a converter that is not storing oxygen properly, even though a dedicated catalyst code may not be present yet.
Wiring harness damage near the exhaust manifold or under the vehicle is a common cause that is frequently overlooked, especially on higher-mileage vehicles where heat and vibration take a toll.
A contaminated sensor from oil or coolant consumption can cause a low voltage signal without completely failing, so inspecting the sensor tip for deposits can provide a clue before replacement.
Some vehicles use a wide-range oxygen sensor for the downstream position, and the generic P0157 definition may not perfectly match the actual circuit behavior, so always consult the factory service information for the specific vehicle.

Can I keep driving with P0157?

The vehicle is generally safe to drive with this code because it does not affect engine operation severely, but you should avoid ignoring it for long periods as it can mask a lean condition that may damage the catalytic converter over time. Have the fault diagnosed promptly, especially if you notice drivability symptoms or a drop in fuel economy.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.