Reliability is one of the most important things people look for when purchasing a vehicle. Your first thought is, how long can I drive my vehicle with only regular maintenance?

However, we all know that eventually, a part will break. Fortunately for everyone, modern cars are fitted with a large number of sensors, each monitoring a specific part of the car. Once one of these sensors registers that a part has gone bad, it will throw an error triggering a check engine light code. Trouble codes allow you to pinpoint which system is failing, shortening diagnosis times. Today we’ll talk about one such code, the P0137. .

It is a universal trouble code that has to do with O2 sensors and your car’s emissions control system. In this guide, we’ll explain what this code is, list its common symptoms, explain how you can diagnose it and how to potentially fix it on your own. 

What is the P0137 Code, and What Does It Mean? 

The P0137 code is a O2 sensor code. In short, it means the voltage signal the oxygen sensor sends is abnormally low. In vehicles with two cylinder banks (V6, V8, some I6), this code refers to bank 1. 

What is an O2 Sensor? 

An oxygen sensor measures the amount of oxygen exiting the engine. The ECU (Engine Control Module) uses this information to adjust the air-fuel mixture. Your vehicle uses two or more sensors for this measurement, one before the catalyst and one after. Without these sensors, the ECU is left without an important piece of the puzzle that can usually affect how the car runs and how fuel efficient it is. 

Is the Bank 1, Sensor 2 Upstream or Downstream? 

The Bank 1 Sensor 2 O2 unit is positioned considered downstream. The upstream sensor is the one that is positioned just before the catalyst, near the exhaust manifold and the downstream one is found right after the cat. P0137 refers to sensor two, the downstream sensor.

A foolproof method of identifying the correct sensor is to start at the first cylinder and to work from there. Cylinder 1 is the furthest forward cylinder and is adjacent to the timing cover. Contrary to popular belief, inline 6 motors can have two banks as well, where the first three cylinders are considered Bank 1 and the second set of cylinders is considered bank 2. 

What Are The Causes of P0137 Code? 

The O2 sensor plays a crucial role in the proper running of your engine. Although sensor failure is rare, related components failing can also cause the P0137 code to be logged, some of those are:

• Faulty O2 sensor
• Exhaust damage
• Failed catalytic converter
• ECU issues
• Wiring harness failure

Faulty O2 Sensor

Manufacturers design O2 sensors to withstand the elements but like any other component on a car that sees a lot of use, hey can fail. Both sensors work together to measure the oxygen content before and after the exhaust passes through the catalytic converter. The ECU compares the voltage readings of both. In the case of the P0137, sensor two’s voltage reading is abnormally low.

A common failure responsible for this code is the inner heater core burning out. The measurement process involves a ceramic bulb that creates its own voltage when exposed to exhaust gas. It can only do this when the sensor is at about 600 °F.

Exhaust Damage

Foreign air entering the exhaust can cause the voltage reading to be lower than usual. Exhaust fumes exiting through an opening can also cause the code. This includes a cracked exhaust manifold. 

It is also possible that damage inside the engine causes similar issues. A vacuum leak can lead to a lean running condition. 

Failed Catalytic Converter

Manufacturers design catalytic converters to survive the extended life of a vehicle, thus, the engine usually experiences issues before the catalyst does. Often unburnt fuel or lingering engine issues damage the catalytic converter.

Among the issues this causes is the P0137. In this context, the P0137 code can be triggered because the cat is not doing its job processing the exhaust fumes. The O2 sensor registers that there is not enough oxygen coming into the engine. The PCM then adjusts the air-fuel mixture based on this, which results in a leaner mixture.

Wiring Harness Failure

This is the most common source of a P0137 code. Rodents or stones can damage the wires and that leads to voltage loss. The sensor connector can also become damaged. The heat O2 sensors operate under can melt the connector or corresponding wires.

ECU Issues

If the PCM malfunctions, it can cause a host of trouble codes, including the P0137. Often a software issue can cause a specific component to malfunction, so you won’t know until you’ve gone through a whole checklist of issues. However, ECU failures are rare.

What Are the Common Symptoms of P0137 Fault Code?

If we look at the various issues that can cause a lean air-fuel mixture, most of them stem from a leak or crack from which air can enter and exit. So it makes sense that the symptoms of a P0137 code would be related to that. Thus, the common symptoms are:

• Check Engine Light
• Leaking Exhaust fumes
• Power loss
• Rough Idle

Check Engine Light

As with any trouble code, the check engine light is one of the first indicators that something is wrong. A P0137 code may cause your light to flash rather than just illuminate. This is because the mismatched air-fuel ratio can cause a bunch of serious issues.

Leaking Exhaust Fumes

One cause of a P0137 is damage to the exhaust. As a result, one of the telltale signs of this issue is the smell of exhaust fumes coming from your vehicle. An even more pertinent sign can be exhaust fumes leaking into the passenger cabin.

Power Loss

Engines are designed to operate using a specific air-fuel ratio. In the case of a lean air-fuel mixture, it can cause power loss. 

It is rare that you will get to experience this symptom as the ECU will register a P0137 within 2 minutes of part failure and adjust the fuel trim to compensate. 

Rough Idle

Of course, a rich air-fuel mixture has its own set of problems. Stalling or poor starting behavior is likely, as the excess fuel cannot ignite. Rough idle is always an indicator that the engine is having trouble figuring out the optimal combustion parameters. 

How to Diagnose and Fix the Cause of P0137 Code 

A flashing check engine light will make any car owner panic. As unnerving as this can be, keeping calm and categorically diagnosing your vehicle is the best way to go about it. The thing about a CEL, even a flashing one, is that it can mean just about anything. Because of that, the first thing you need to do is pull out your trusty OBD II Reader and run a full diagnostic scan of your car. 

ODB II Reader

A scan tool should always be your go-to initial diagnosis tool when a CEL pops up. An ODB II reader allows you to interact with the ECU of the vehicle and read all the codes that have been logged. 

You’ll be able to tell if the codes are current or history codes, whether they are serious or not. Best of all, most scanners will let you monitor the live data from your ECU, which can also help when it comes to setting a diagnosis. To start, plug it into OBD II port and start the scan. Consult your owner’s manual for the exact location of the port since it differs from car to car. 

As well as being able to identify the P0137 code, you can also tap into the live oxygen sensor data. The downstream sensor we’re looking at should have high peaks and dips, but the gradient will be slower.

Visual Inspection 

The most common causes of a P0137 are external. So, it only makes sense you should look around your vehicle for damage before pursuing more invasive diagnostic techniques.

It would be best to look at your exhaust system under a lift, but jacking it up and checking the exhaust system will work just as well. Inspect your sensor, the catalytic converter, and exhaust manifold for damage as well. Internal cat damage usually presents with other trouble codes.

The nature of exhaust material means that there is unlikely to be a small leak, rather it will be loose altogether, or the holes will be large. This is the same for cracks in the manifold.

Wiring Harness Damage

As the O2 sensor is one of the likely causes of the P0137 code, it makes sense that a break in the circuit can lead to low voltage. The ECU or the wiring harness issues can cause this, but the latter is more likely. 

A loose or damaged connector is a common cause of the P0137. Look for any external damage to the wiring harness or sensor connector. A common sign is melted plastic near the end of the sensor or signs of burning, like black soot.

Multimeter Test

This test can be conducted with either a multimeter or an oscilloscope. An oscilloscope will allow you to measure the signal voltage and wavelength, but most car owners have a multimeter on hand.  

The O2 sensor looks slightly different compared to many of the others that you’ll diagnose in your car’s lifetime. It still uses the three-wire configuration for the harness, but there are two additional wires. These wires are the positive and negative heater wires.

The ground (brown), signal wire (red) and live wire (green) enter through the top of the sensor, which links up to a separate connector that attaches to the wiring harness. To diagnose the sensor, touch your positive probe to the signal wire and the negative probe to the nearest ground on your car.

Once you’ve probed your sensor, it should give you between 0.2 and 0.8 volts. The numbers do somewhat differ based on the sensor manufacturer, but since the P0137 code is specifically for low voltage, that’s what you should be on the lookout for.

Vacuum Leaks

Even though it is easier to recognize holes in the exhaust that may lead to vacuum leaks, it may be considerably more difficult to detect them if they are coming from the engine. This can also cause your engine to run lean, which throws the P0137 code.

A vacuum tester will help identify whether you have a vacuum leak. Attach your vacuum tester to any vacuum hose in the engine. At engine idle, your tester gauge should show around 14 to 22 HG. If the pressure is fluctuating or too low, it means you have a leak.

There are many DIY methods of finding exactly where the leak is. The safest way is to use a smoke machine, but not everyone has this type of equipment at hand. The next best thing is to start the engine and then spray the suspected vacuum lines with a starter spray. If the idle normalizes, you’ve found your leak. That being said, starter sprays are flammable so do this at your own risk.  

Does P0137 Clear Itself?

While the P0137 code can clear itself, it’s best that you clear it manually. This will save you a lot of time and hassle. If you don’t have access to one, the code should clear itself within several drive cycles. The problem with this is that you won’t know whether you’ve fixed the issue for quite a while as drive cycles usually mean driving the car for a certain distance, at a certain RPM. 

Is It OK to Drive With P0137?

Many cars will flat out not allow you to drive with a P0137 or limit your speed. However, even if your car does not have limp mode, you should not risk driving around with an active 02 sensor issue.

You might end up having to replace a catalyst or exhaust manifold, but if left unchecked, you can destroy your engine.

Fix the P0173 Code Quality Parts!

Getting the P0173 code sorted out means doing a good job with diagnosing the issue, but also finding the right parts for your car. Here at eEuroparts.com, we offer one of the largest catalogs of quality parts for European vehicles you’ll find online. Just pick your vehicle from the dropdown menu, select the parts you need, and we’ll have them shipped to your door in no time!