Error Code P0157 is defined as O2 Sensor Low Voltage Bank 2 Sensor 2. This refers to a trouble in the Bank 2 Sensor 2 of the O2 (oxygen) sensor, which is located near the backside of the exhaust, behind the catalytic converter. The code is set when the PCM (powertrain control module, also known as ECM or engine control module in some makes) detects the voltage signal of the oxygen sensor remains low for an extended period of time.
This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II, or made from 1996 up to present. Specifications on the definitions and details on troubleshooting and repair of course, differ from one make/model to another.
This error code refers to the O2 sensor behind the catalytic converter, at the back side of the exhaust. The main function of this sensor is to regulate the amount of oxygen that gets inside the catalytic converter. Lean mixture of O2 and gas means there is too much oxygen in the exhaust, while a rich mixture of O2 and gas means there is not enough oxygen in the exhaust.
The catalytic converter works by helping tailpipe control the emissions and ‘clean up’ the exhaust. The PCM then uses the signal from heated O2 sensor to the rear catalyst to check the efficiency of the catalytic converter by comparing it in the O2 sensors in front of the catalyst.
When the PCM sees there is very little or no difference between the quality of the exhaust in post and pre-catalyst sensor, it will conclude that the catalytic converter is having trouble. To simplify, the job of the post catalytic converter sensor it to monitor the catalyst efficiency, and NOT control fuel, though it can influence through the operation of the O2 sensor.
There should be a perfect balance in the mixture level of oxygen and gas, as neither lean nor rich is good for the catalytic converter as both can cause it to fail prematurely. The O2 sensor switches back and forth from lean to rich fast. Error Code P0157 is set when the PCM determines that the O2 sensor has been in low voltage longer than specified time. This means the signal voltage Bank 2 Sensor 2 of the O2 sensor is below normal.
Check Engine light is the most obvious symptom for Error Code P0157. It will not usually cause drivability issues, since it only suggests an input that measures the efficiency of the catalyst (unlike the front Oxygen sensors). However, there’s a good chance that other error codes may be present as well, such as lean condition codes or pre-O2 sensor codes, which may cause drivability issues, such as the Error Code P0143. For some cases with symptoms, the common symptoms will be:
- Engine performance problems
- Rough running
- Poor idle
- Engine running rich (happens when the PCM tries to compensate by adding fuel for low oxygen sensor voltage reading)
- Engine running lean (happens when the vacuum is large enough that PCM will not be able to compensate even by adding fuel)
- Decrease in fuel economy due to PCM in “limp home” mode
If you suspect your vehicle is running lean, then it is crucial to fix that error and clear the code and check whether the P0157 comes back. Engine running lean could also register lean codes such as P0171 and P0174. Other possible causes of P0157 include:
- Exhaust air leak in front of O2 sensor which gives false reading
- Engine running lean
- Corroded or damaged oxygen sensor
- Shorted to ground oxygen sensor signal circuit
- Open ground circuit of oxygen sensor
- Failed O2 sensor (could be contaminated with coolant or fuel)
- Damaged PCM (rare)
How to Check
Use the OBD-II scanner to collect the freeze frame data of the trouble code. Then, use the scan tool to check the voltage of the O2 sensor to see if it switches properly.
If the trouble code is present, then check it through the live data to see if the engine coolant temperature sensor still works properly. Then, check the pressure of the fuel through the pressure gauge. It should be at 40 psi.
Check the exhaust for any signs of leaks, especially in the pipes and manifold.
Check the rear O2 sensor wires for breaks, shorts and exposed wires.
In some cases, the computer may need to be updated from the dealer. Also, some vehicles may require advanced diagnosis, particularly if there is already a problem in the PCM.
How to Fix
As with other error codes, repairs for P0157 are done by addressing the issues that arise from the diagnosis, such as:
- Replacement of the rear O2 sensor
- Repair the cause of the fuel pressure abnormality
- Repair broken circuits and shorted wires to the O2 sensor
- Repair faulty engine coolant temperature sensor
- Replace or update PCM, if necessary
Error Code P0157 is a moderately serious trouble code. It can definitely cause drivability issues, which can be dangerous when driving at highway speed. Also, if not addressed properly and in a timely manner, it can cause serious damage to catalytic converter, which can be quite expensive to fix.