P0137 – O2 sensor circuit low voltage

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When the voltage from your downstream oxygen sensor is too low, the P0137 code will activate. Low readings from oxygen sensor 2 indicate the air-to-fuel ratio after the catalytic converter is too rich. This could be caused by wiring issues, vacuum leaks, and other problems.

While a faulty oxygen sensor may be causing this code, it’s not always the culprit. Read on below to learn how to correctly diagnose a P0137 trouble code before it leads to further engine damage. 

P0137 code definition

P0137 code definition (generic): Oxygen O2 sensor circuit low voltage (bank 1, sensor 2)

P0137 Subaru code definition: O2 sensor circuit low voltage (bank1 sensor2)

P0137 Toyota code definition: O2 sensor circuit low voltage bank 1 sensor 2

Here's where you can get a thorough understanding of the P0137 OBD2 code
If you want to learn about the P0137 OBD2 code, this is the right place

What does P0137 mean?

The number 2 oxygen sensor is downstream of the catalytic converter in your engine. Bank 1 points you toward the side of the engine that contains cylinder 1.

Downstream oxygen sensors measure the air/fuel mixture to make sure it’s maintaining the correct ratio. Ideally, your engine’s air-to-fuel ratio should be 14.7:1 (e.g., 14.7 parts air for each 1 part fuel). The upstream oxygen sensor measures the air going into the catalytic converter, telling it what adjustments to make. 

Fluctuations from the upstream oxygen sensors are normal, but the downstream oxygen sensor should report a steady voltage of around .45 volts. If this drops below a certain threshold (between .04 and .21 volts, depending on the vehicle), the P0137 trouble code will activate. 

Usually, the P0137 code won’t trigger unless the sensor reads low for more than 20 seconds. The timing can vary depending on your make and model, though.

P0137 is a generic code, so the definition is the same across vehicles. The exact fix can vary depending on your car’s manufacturer, however. Keep your repair manual handy as you’re going through your diagnosis if your vehicle has specifically recommended fixes. 

What are the symptoms of the P0137 code?

In some cases, there are no drivability issues associated with the P0137 trouble code. If there are symptoms, they include:

  • Activation of the check engine light
  • Strong odor in the exhaust
  • Rough idle
  • Engine running rich

What are the causes of P0137?

The P0137 code notifies you of the voltage lowness from oxygen sensor.
When the voltage from your downstream oxygen sensor is too low, the P0137 code is triggered.
  • Faulty oxygen sensor
  • Clogged catalytic converter
  • Exhaust leaks around the oxygen sensor
  • A short or open signal in the oxygen sensor circuit
  • Fuel pressure too high or too low

How serious is the P0137 code?

The P0137 trouble code is of moderate severity. You can drive for a short time while the code is active without danger to the driver. Long-term operation with a rich air-to-fuel ratio can damage your engine, however. Even if you’re not experiencing drivability symptoms, you should repair the issue as soon as possible. 

How to diagnose and fix the P0137 code

Tools you’ll need:


  1. Check for any technical service bulletins for your vehicle related to the P0137 trouble code. If there are, follow those instructions before conducting the generic diagnosis below. 
  2. Use an OBD2 scan tool to check for other trouble codes. Codes related to the exhaust system or air/fuel mixture can help you identify the root of the P0137 code.
  3. Inspect the wiring around the downstream oxygen sensor, as well as the wiring harness. Replace any wires that are corroded or damaged. Check all the connections, as well, and ensure they’re secure and uncorroded.
  4. Turn on your car without starting the engine. Use the OBD2 scanner to read the live data from all the oxygen sensor terminals. Check that both the ground and signal reference to ensure they’re at least .3 volts.
  5. Check all vacuum tubes and valves of the exhaust system for leaks. You can do this visually, feeling any hidden areas for damage or wear. You can use a smoke machine to identify leaks invisible to the naked eye for a more thorough diagnosis. 
  6. Test your oxygen sensor with a multimeter. Touch the black lead to the battery’s negative terminal and the red lead to the oxygen sensor signal wire. When the engine is off, the sensor should read somewhere between .1 and 1 volts. Remove the multimeter and turn on your engine, allowing it to warm up. Test the sensor again. The voltage should change. If it doesn’t, the sensor is likely defective. 
  7. Used a vacuum gauge to test for clogs in the catalytic converter. Connect the vacuum gauge to a vacuum hose at the intake manifold, then start your engine and let it idle until it’s warm. Check the vacuum reading; it should be between 18 and 22 in-Hg. Ensure your car is in park with the emergency brake on, then increase the RPM to around 3,000. The vacuum reading should drop briefly before returning to its previous reading. If it doesn’t, you likely have a blockage.
  8. Take your car for a hard drive, which can clear blockages in the catalytic converter and those elsewhere in your system. To do this, find a road where you can both reach high speeds and change speeds safely, such as a highway in the late night or early morning hours. Drive a few miles at 55-60 mph, then quickly reduce speed to around 35-40 mph. Repeat this process a few times.

Common mistakes to avoid while diagnosing the P0137 code

Many mechanics replace the downstream oxygen sensor before checking for other issues. Conduct a full diagnosis of the circuit, valves, and other nearby components before making any replacements. 

How to fix P0137 Code.

Tips to avoid P0137 in the future

Vacuum leaks and damaged wiring can lead to several serious engine problems. Treating your engine with an anti-corrosion coating can help to preserve these components. You can also be proactive in identifying issues. Check the wires and hoses periodically, ensuring they’re well-connected and away from engine components that could damage them.

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