What Does P0157 Mean?
The downstream O2 sensor on Bank 2 (Sensor 2, post-catalytic converter) is reporting a persistently low voltage, indicating it is stuck reading a lean exhaust condition. The voltage remains below the expected minimum for longer than the PCM allows. This could indicate a sensor fault, a wiring issue, or an exhaust leak introducing air downstream of the converter. The driver typically won't notice driveability symptoms.
Common Causes
35%
Failed downstream O2 sensor stuck producing low voltage
25%
Exhaust leak downstream of the catalytic converter near the sensor location
20%
Open or high-resistance signal wire causing voltage to read low
10%
Catalytic converter over-efficiency or unusual exhaust chemistry
10%
Signal wire shorted to ground pulling voltage low
Diagnostic Steps
1
Monitor Bank 2 Sensor 2 voltage on live data — a stuck reading near 0V indicates a short to ground or dead sensor; near 0.45V indicates a possible open circuit showing PCM bias voltage.
2
Disconnect the O2 sensor and check the PCM side for bias voltage (approximately 0.45V key on, engine off) — if present, the PCM circuit is functional.
3
Inspect the sensor wiring for damage, chafing against exhaust components, and corrosion at the connector — measure signal wire resistance to ground (should be very high, not near 0 ohms).
4
Check for exhaust leaks between the catalytic converter and the sensor by running the engine and feeling for escaping gases or using a smoke machine.
5
Reconnect and perform a propane enrichment test at the tailpipe — if the sensor responds by going above 0.6V, it is functional and the issue is likely an exhaust leak or wiring problem.
Estimated Repair Cost
$100 - $350
Parts + labor, varies by vehicle and location
The downstream O2 sensor on Bank 2 (Sensor 2, post-catalytic converter) is reporting a persistently low voltage, indicating it is stuck reading a lean exhaust condition. The voltage remains below the expected minimum for longer than the PCM allows. This could indicate a sensor fault, a wiring issue,...
The most common cause of P0157 (Oxygen Sensor Low Voltage, Engine Bank B, Downstream of Catalytic Converter) is: Failed downstream O2 sensor stuck producing low voltage
Typical repair costs for P0157 range from $100 to $350, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to continue driving. This downstream sensor monitors catalyst efficiency and does not directly control fuel mixture. The MIL will be on and the vehicle will not pass emissions testing.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0157 to identify the root cause.
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Quick Info
Category
Powertrain
System
Emissions / Exhaust System
Difficulty
Type
Generic (SAE)
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