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P0157 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.

Definition

When the P0157 trouble code pops up, it means that the bank two sensor two oxygen sensor is detecting a low voltage.

Meaning

The oxygen sensor in question is located behind the catalytic converter and to the rear of the exhaust. This sensor's primary function is to control the amount of oxygen that comes from the catalytic converter. 

If there's too much oxygen in the exhaust, it's referred to as a lean mixture. When there's not enough oxygen in the exhaust, it's called a rich mixture. If the mixture is lean, the sensor will register a low voltage. When the mixture is rich, the sensor will record a high voltage.

Both states are problematic for the catalytic converter, the function of which is necessary for your vehicle to comply with federal environmental regulations. 

Causes

The most common cause of the P0157 code is a faulty oxygen sensor, which might mean that there's nothing wrong with the oxygen level in your exhaust. This is not the only reason you might be getting this particular trouble code, though.  

There may be a short on voltage to the oxygen circuit. Alternately, you might have an exhaust leak, which can throw the sensor off. Your fuel pressure may also be low. 

In some cases, the code might be indicating the correct problem if your engine is running lean. This means that the combustion occurring in your engine isn't as efficient as it could be.

Ideally, you want the amount of fuel to be perfectly balanced with the amount of air in the cylinder, so there's no waste. If your engine is running lean, you have too much air or not enough fuel, which the oxygen sensor will detect in the form of high amounts of oxygen in the exhaust. 

Symptoms

Some of the symptoms of the issues that might cause this code will be reasonably apparent, including when:

  • The check engine light is on.
  • You smell exhaust, which might be due to a possible leak.
  • Your engine runs rough.
  • You notice a lack of power.

There is no guarantee that these symptoms will be present. There may not be any sign that your vehicle has an issue, so regular maintenance is essential. 

Diagnosis

The first step to diagnosing the issue is to collect the trouble code and freeze frame data from the OBD-II scanner so that you can narrow down the exact problem. 

From there, you or your mechanic will use a scan tool to see if the oxygen sensor voltage is switching the way it should. You or your technician should also use a fuel pressure gauge to check your fuel pressure, which should be around 40 PSI. 

The exhaust system should always be inspected for leaks when this trouble code occurs. You or your mechanic should check the wires connected to the oxygen sensor in question for breaks or shorts. 

If an engine coolant temperature sensor trouble code is present, someone will need to view the sensor's live data to ensure it is working correctly.

The dealer may need to update the computer, and advanced diagnostics could also be necessary to see if there's an issue with the Primary Control Module (PCM).

Common mistakes

The most common mistake is simply replacing the oxygen sensor without checking for loose connections or exhaust leaks. While the oxygen sensor isn’t an overly expensive part, you don't want to waste your time or money on something that isn't going to solve your problem. 

How serious is this?

The P0157 trouble code is a relatively serious issue. Don't ignore it. It can cause drivability issues, and there is a chance that your vehicle could stall out. If you don't address this issue, it may result in damage to your catalytic converter. That's an expensive fix, so we recommend addressing this trouble code as soon as possible.

What repairs can fix the code?

Replacing the rear oxygen sensor might fix this code, but you should not do this without looking for exhaust leaks or loose connections. You may also need to repair broken circuitry or shorted wires that are connected to the oxygen sensor.

Another issue might be present that explains the trouble code, but if your fuel pressure is low, you should look for and address any abnormalities that might be causing it. 

If an engine coolant temperature sensor code is also present, you will probably need to replace or repair the sensor. Finally, you might need to update or replace the PCM. The necessary repairs will depend on the underlying issues, and costs will vary accordingly.

Related codes

None listed.

Conclusion

If you notice any of the symptoms described above, it's vital that you address them to avoid expensive damage and potentially dangerous situations if your vehicle stalls. The sooner you find and fix the issue causing the code to be shown, the better off you will be.