What Does P1157 Mean?
This code indicates the downstream heated oxygen sensor (HO2S) on bank 2 is not switching as expected and is stuck indicating a lean condition. The post-catalyst sensor should show a relatively stable voltage near 0.6-0.7V when the catalyst is functioning properly. A fixed lean reading (low voltage) downstream suggests either the sensor is biased lean due to an internal fault, or actual lean exhaust conditions are present after the catalyst on bank 2.
Common Causes
35%
Failed downstream HO2S on bank 2 (internally biased lean or contaminated)
25%
Exhaust leak between the catalytic converter and the downstream sensor on bank 2
20%
Degraded catalytic converter on bank 2 allowing excess oxygen pass-through
12%
Wiring issue causing high resistance in the sensor signal circuit
8%
Persistent lean fuel condition on bank 2 overwhelming catalyst capacity
Diagnostic Steps
1
Monitor the bank 2 downstream HO2S voltage with live data at steady cruise; a healthy sensor behind a good catalyst should hold 0.5-0.7V with minimal fluctuation.
2
Perform a propane enrichment test near the downstream sensor to verify the sensor can respond to a rich condition; if voltage rises, the sensor is functional.
3
Inspect for exhaust leaks at the catalyst-to-exhaust pipe connection and around the downstream sensor bung on bank 2.
4
Check bank 2 upstream fuel trims; if long-term fuel trim is significantly positive, address the upstream lean condition first.
5
If the sensor fails to respond to enrichment, replace the bank 2 downstream HO2S.
Estimated Repair Cost
$120 - $400
Parts + labor, varies by vehicle and location
This code indicates the downstream heated oxygen sensor (HO2S) on bank 2 is not switching as expected and is stuck indicating a lean condition. The post-catalyst sensor should show a relatively stable voltage near 0.6-0.7V when the catalyst is functioning properly. A fixed lean reading (low voltage)...
The most common cause of P1157 (Lack of Downstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 2) is: Failed downstream HO2S on bank 2 (internally biased lean or contaminated)
Typical repair costs for P1157 range from $120 to $400, depending on the vehicle, location, and whether you do it yourself or go to a shop.
No direct safety hazard for the driver. However, a stuck lean downstream sensor can mask catalytic converter deterioration, leading to elevated tailpipe emissions and potential emissions test failure.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1157 to identify the root cause.
OBDHut Mobile App
Scan codes directly from your car with the OBDHut app.
Coming Soon
Quick Info
Category
Powertrain
System
Fuel / Air Metering (Catalyst Monitoring)
Difficulty
Type
Manufacturer
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
