If your check engine light is on and you see a P0137 code on your scanner, you’re likely dealing with a low voltage signal from your car’s oxygen sensor. Understanding what causes a P0137 code is the first step to fixing it and getting your vehicle running smoothly again. This code specifically points to the downstream oxygen sensor, also called the post-catalytic converter sensor or Bank 1, Sensor 2. It’s telling you that the sensor’s voltage output is staying too low for too long, which means your car’s computer is getting a reading of a too-lean exhaust mixture on that side of the engine.
While it might seem minor, ignoring this code can lead to poor fuel economy, reduced engine performance, and even damage to your catalytic converter over time. Let’s break down exactly why this happens and what you can do about it.
What Causes A P0137 Code
The P0137 code stands for “O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2).” To get what that means, you need to know a little about how these sensors work. The downstream oxygen sensor, located after the catalytic converter, monitors the converter’s efficiency. It compares the oxygen levels before and after the exhaust gases pass through it.
A healthy sensor fluctuates its voltage signal between roughly 0.1 and 0.9 volts. A persistent low voltage signal—typically below 0.45 volts for an extended period—triggers the P0137 code. This low voltage reading tells the engine computer (PCM) that the exhaust stream is too lean, meaning it has too much oxygen or not enough fuel. The root cause can be a problem with the sensor itself, the exhaust system, or even issues delivering fuel.
Common Culprits Behind the P0137 Code
Here are the most frequent reasons why you might be seeing this diagnostic trouble code:
* A Faulty Downstream Oxygen Sensor (Bank 1, Sensor 2): This is the most common suspect. The sensor can simply wear out over time, get contaminated by oil or coolant leaks, or become coated with soot from a rich-running engine. Internal heater circuit failures are also common, preventing the sensor from reaching its optimal operating temperature quickly.
* Damaged or Corroded Wiring and Connectors: The wires leading to the O2 sensor are in a harsh environment—exposed to heat, road salt, and physical damage. Frayed wires, melted insulation, or corroded pins in the electrical connector can create a poor connection, leading to a false low-voltage signal.
* Exhaust Leaks: A leak in the exhaust pipe, especially near the sensor or between the engine and the converter, can allow outside air to be sucked into the exhaust stream. This extra oxygen fools the sensor into reporting a lean condition, even though the fuel mixture might be correct.
* A Failing Catalytic Converter: While less common as a direct cause, a severely degraded or clogged catalytic converter can alter exhaust flow and gas composition, potentially leading to abnormal sensor readings that trigger this code.
* Fuel System Issues: Problems that create an actual lean condition can also set this code. This includes a weak fuel pump, clogged fuel filter, dirty fuel injectors, or a fuel pressure regulator that’s not working right. If the engine is truly running lean, both the upstream and downstream sensors may show lean readings.
* Vacuum Leaks: Unmetered air entering the engine through a cracked vacuum hose, a bad intake manifold gasket, or a faulty PCV valve leans out the air/fuel mixture. This real lean condition will be detected by the oxygen sensors.
Step-by-Step Diagnostic Guide
Before you just throw a new oxygen sensor at the problem, it’s smart to do some basic checks. This can save you time and money. Always ensure the engine is cool before working near the exhaust system.
Step 1: Check for Other Codes
First, use your OBD2 scanner to see if there are any other codes stored alongside P0137. Codes like P0171 (System Too Lean) or P0300 (Random Misfire) can point to a fuel or vacuum issue affecting the oxygen sensor reading. Addressing those first might resolve the P0137.
Step 2: Visual Inspection
This is a crucial and often overlooked step. Safely raise the vehicle and visually inspect the following:
* Locate the Bank 1, Sensor 2 oxygen sensor. Remember, Bank 1 is the side of the engine with cylinder #1.
* Carefully examine the sensor’s wiring harness. Look for any signs of chafing, melting, or cuts.
* Check the electrical connector. Is it fully seated and locked? Are the pins clean and free of green or white corrosion?
* Inspect the exhaust manifold, pipes, and the catalytic converter itself for any signs of holes, cracks, or severe rust that could cause a leak.
Step 3: Test the Oxygen Sensor Signal
Using a scan tool that can read live data, you can watch the sensor’s voltage in real-time. With the engine fully warmed up, observe the voltage for Bank 1, Sensor 2.
* A good downstream sensor will usually show a fairly stable voltage, often hovering around 0.6-0.7 volts, with slow fluctuations. It should not mirror the rapid swings of the upstream sensor.
* If the live data shows a stuck low voltage (e.g., constantly at 0.1 or 0.2 volts), it strongly suggests a bad sensor or wiring issue.
* You can also perform a manual test with a digital multimeter set to DC volts to check the signal wire, but using scan tool data is often easier.
Step 4: Check for Exhaust and Vacuum Leaks
Listen for a hissing or tapping sound near the engine while it’s running, which can indicate a vacuum leak. For exhaust leaks, you might hear a puffing or rumbling sound from under the car. A mechanic might use a smoke machine to precisely find vacuum leaks or listen with a stethoscope for exhaust leaks.
How to Fix a P0137 Code
Once you’ve identified the likely cause, you can proceed with the repair. Here’s a general guide:
1. Fixing Wiring Issues: If you found damaged wires, they can sometimes be repaired with high-temperature solder and heat-shrink tubing. For corroded connectors, electrical contact cleaner and a small brush can work wonders. Ensure the connection is solid and waterproof after your repair.
2. Replacing the Oxygen Sensor: If the sensor is faulty, replacement is the solution. Use an OEM or high-quality sensor. Remember to apply anti-seize compound only to the threads of the new sensor, avoiding the sensor tip. Over-tightening can damage it, so use a proper oxygen sensor socket and follow torque specs.
3. Repairing Exhaust Leaks: Small holes can sometimes be patched with exhaust repair tape or paste as a temporary fix, but welding or replacing the affected exhaust section is the proper, permanent repair.
4. Addressing Fuel or Vacuum Problems: Fix any identified vacuum leaks by replacing hoses or gaskets. If fuel system issues are suspected, testing fuel pressure and inspecting injectors may be necessary. These repairs can be more involved and might require a professional’s help.
After making the repair, clear the diagnostic trouble codes with your scanner. Take the car for a test drive that includes various speeds and engine loads to allow the computer to run its self-tests. The goal is to complete a “drive cycle” and see if the check engine light and P0137 code return.
FAQs About the P0137 Code
Q: Can I drive my car with a P0137 code?
A: You can usually drive for a short time, but it’s not recommended for the long term. You may experience worse gas mileage and higher emissions. The main risk is that a real lean condition (if that’s the cause) can damage the engine, or a faulty sensor can mask a failing catalytic converter.
Q: How much does it cost to fix a P0137 code?
A: Costs vary widely. A DIY oxygen sensor replacement might cost $50-$200 for the part. At a shop, with labor, you could be looking at $200-$400. If the fix involves finding a vacuum leak or repairing exhaust pipes, the cost will be higher.
Q: Is a P0137 code serious?
A: It’s moderately serious. It won’t typically leave you stranded, but it should be addressed within a few weeks to prevent other problems and restore optimal engine performance and efficiency.
Q: What’s the difference between Bank 1 and Bank 2?
A: Bank 1 refers to the side of the engine that contains cylinder #1. Bank 2 is the opposite side. In a straight 4- or 6-cylinder engine, there is only one bank (Bank 1). V6 and V8 engines have two banks. Sensor 2 always means the downstream sensor, after the catalytic converter.
Q: Could it be the catalytic converter?
A: While possible, the catalytic converter is less often the direct cause of a P0137. More often, a bad converter will set different codes like P0420 (Catalyst System Efficiency Below Threshold). However, a severely clogged converter can disrupt exhaust flow enough to affect sensor readings.
Dealing with a P0137 code is very manageable with a systematic approach. Start with the simple, free checks like looking at the wiring and listening for leaks. Use a scan tool to look at live data if you can. This will point you toward the real issue, whether it’s a simple sensor replacement or a more elusive vacuum leak. Remember, the code is a symptom; your job is to find the underlying cause to make a lasting repair. Taking care of it promptly will help keep your fuel costs down and protect your engine from potential harm.