P0157 – Meaning, Causes, Symptoms, & Fixes

Code P0157 Definition

O2 Sensor Circuit Low Voltage (Bank 2, Sensor 2)

What Does P0157 Mean?

Oxygen sensor 2 is the downstream oxygen sensor in relationship to the catalytic converter. It measures the air-fuel ratio coming out of the catalytic converter to ensure the catalytic converter is functioning properly. The catalytic converter works to maintain the stoichiometric air-fuel ratio 14.7:1 while the powertrain control module constantly flip-flops between rich and lean air-fuel mixtures due to the input from the upstream oxygen sensor (sensor 1). Therefore, the downstream oxygen sensor (sensor 2) should produce a steady voltage of approximately 0.45 volts. When trouble code P0157 is set, this indicates that there is low voltage (below 400 millivolts) for more than 20 seconds or the air fuel ratio remained too lean for too long in Bank 2.

P0157 Symptoms*

  • Check Engine Light
  • Possible exhaust leaks
  • Exhaust odor
  • Vehicle running idle/rough

*No adverse conditions in some cases

P0157 Causes

  • Faulty oxygen sensor
  • Short on voltage to O2 circuit
  • Exhaust leaks
  • Fuel pressure very high or low
  • Engine running very rich or lean

Code P0157 Severity – Moderate   

Driving with trouble code P0157 is acceptable but only for a short period of time. Driving with this code for an elongated period of time could result in greater engine damage.

Code P0157 Common Diagnosis Mistakes

Replacing the O2 sensor without checking for any leaks or loose connections or for any voltage issues within the  O2 signal circuit.

P0157 Diagnosis

Difficulty of Diagnosis and Repair – (1-4)

  1. Visually check the O2 sensor wiring and harness connections for any corrosion in the connections,or for any physical damage that might be present.
  2. Check live data from all 4 terminals of O2 sensors. Make sure your key is on and your engine is off when checking this. You are checking for ground and signal reference. As long as you are getting ground and .3 volts of signal reference.
  3. Check for exhaust leaks before the sensor and repair as needed
  4. If these checks do not clear your Check Engine Light you may need to see a mechanic so they may conduct specific pinpoint tests for further diagnosis.
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