If your check engine light is on and the code reader says P0139, you’re probably wondering what it means for your car. What Does P0139 Honda Accord Mean & What You Can Do About It? Simply put, it’s a diagnostic trouble code indicating a slow response from your vehicle’s Bank 1, Sensor 2 oxygen (O2) sensor. This sensor monitors the exhaust gases after they’ve passed through the catalytic converter, and a delay in its signal can cause performance issues.
This code is common in many Honda Accord model years, particularly the 6th and 7th generations (1998-2007). While it might not cause your car to break down immediately, ignoring it can lead to poor fuel economy, failed emissions tests, and potential damage to the catalytic converter—an expensive part to replace. This guide will explain the causes, symptoms, and practical steps you can take to fix the P0139 code yourself.
What Does P0139 Honda Accord Mean & What You Can Do About It?
The P0139 code is defined as “O2 Sensor Circuit Slow Response (Bank 1, Sensor 2).” Let’s break down what each part of that means:
* Bank 1: This refers to the side of the engine that contains cylinder number 1. In most Honda Accords with inline 4-cylinder engines, there is only one “bank.” In V6 models, Bank 1 is typically the front bank of cylinders.
* Sensor 2: This specifies the downstream oxygen sensor. Sensor 1 is always upstream (before the catalytic converter), and Sensor 2 is downstream (after the catalytic converter).
* Slow Response: This means the sensor’s voltage signal is not changing quickly enough when compared to the upstream sensor’s activity. The downstream sensor should show a relatively stable signal; if it’s mimicking the upstream sensor too slowly, the car’s computer (PCM) triggers P0139.
The primary job of the downstream O2 sensor is to monitor the efficiency of the catalytic converter. A slow response means it can’t do its job properly, leading the PCM to use default fuel settings that aren’t optimal.
Common Symptoms of a P0139 Code
You might notice one or more of these signs when the P0139 code is active in your Honda Accord:
* Illuminated Check Engine Light (CEL): This is the most common and often the only symptom initially.
* Slightly Reduced Fuel Economy: Because the PCM can’t fine-tune the air/fuel mixture accurately, you may see a drop in miles per gallon.
* Failed Emissions Test: The code will cause an automatic failure in most states during your vehicle’s inspection.
* Lack of Performance: In some cases, you might feel a slight hesitation or rough idle, though the car will usually remain drivable.
Main Causes of P0139 in a Honda Accord
Several issues can trigger this code. Here are the most frequent culprits, starting with the most likely.
1. A Failing or Lazy Downstream O2 Sensor
This is the number one suspect. Over time, oxygen sensors become contaminated with exhaust byproducts (like sulfur, lead, or fuel additives) and simply wear out. Their internal response time slows down, leading directly to the P0139 code. The sensor may still be producing a signal, but it’s not fast enough for the PCM’s expectations.
2. Exhaust Leaks Near the Sensor
A small leak in the exhaust manifold, a cracked pipe, or a bad gasket before or near the downstream O2 sensor can allow outside air to dilute the exhaust stream. This causes the sensor to read an artificially lean mixture and can drastically alter its response time, confusing the PCM.
3. Issues with the Catalytic Converter
While a failing catalytic converter often throws different codes (like P0420), it can sometimes contribute to a P0139. If the converter is not processing exhaust gases correctly, it can affect the readings the downstream sensor receives, making its response seem slow or erratic.
4. Wiring or Connector Problems
The electrical path from the O2 sensor to the PCM is long and exposed to heat and vibration. Common issues include:
* Chafed, burnt, or broken wires.
* Corrosion or poor contact in the sensor’s electrical connector.
* A damaged harness where it passes near hot exhaust components.
5. Rare PCM Software Glitch
On rare occasions, the problem might be with the Powertrain Control Module itself. A software fault or a failing circuit inside the PCM could cause it to misread a perfectly good sensor signal. This is a last-resort diagnosis after checking everything else.
How to Diagnose and Fix the P0139 Code
Before you spend money on parts, it’s wise to do some basic diagnostics. You’ll need a basic tool set and an OBD2 scanner that can show live data.
Step 1: Confirm the Code and Check for Others
Clear the P0139 code with your scanner and take the car for a 10-15 minute drive that includes both city and highway speeds. If P0139 returns quickly, you have an active problem. If other codes are present (especially any related to the upstream O2 sensor or fuel trim), address those first, as they can cause the P0139.
Step 2: Perform a Visual Inspection
This is a crucial and often overlooked step. Safely raise the car and support it on jack stands.
1. Locate the downstream oxygen sensor. It will be in the exhaust pipe, behind the catalytic converter.
2. Carefully inspect the sensor itself, its wiring, and the connector. Look for obvious damage, melting, or corrosion.
3. Check the exhaust system for any signs of leaks, cracks, or sooty deposits around joints and welds.
Step 3: Check Live Data with a Scanner
With the engine fully warmed up and running, use your scanner to view the live data stream for Bank 1, Sensor 2 (B1S2) oxygen sensor voltage.
* A healthy downstream sensor should show a fairly steady voltage, usually fluctuating slowly between 0.5 and 0.7 volts.
* If the voltage is stuck high (near 0.9V), stuck low (near 0.1V), or is completely flat, the sensor is likely bad.
* If it is mimicking the rapid swings of the upstream sensor, that indicates a faulty catalytic converter or a exhaust leak.
Step 4: Test for Exhaust Leaks
While the engine is cold, start it and listen carefully underneath for any hissing or ticking sounds that change with engine RPM. You can also use a piece of hose held to your ear to help pinpoint leaks. Another method is to carefully spray soapy water on suspected areas while the exhaust is cold; bubbles will form if there’s a leak.
Step 5: Test Sensor Wiring
If the visual check revealed questionable wiring, you can perform a resistance check. Disconnect the sensor connector and the PCM connector. Using a multimeter, check for continuity in each wire between the two ends. Also check for a short to ground. Any break in continuity or unintended ground confirms a wiring fault.
Repair Instructions: Replacing the Oxygen Sensor
If you’ve determined the downstream O2 sensor is faulty, here’s how to replace it.
Tools & Parts Needed:
* New downstream O2 sensor (OEM Honda or a quality brand like Denso, NTK)
* O2 sensor socket (usually 22mm) and a long ratchet/breaker bar
* Penetrating oil (like PB Blaster)
* Basic socket set and jack stands
Step-by-Step Process:
1. Safety First. Ensure the engine and exhaust are completely cool. Park on a flat surface, engage the parking brake, and chock the rear wheels. Lift the car securely and support it with jack stands.
2. Locate and Access the Sensor. Find the downstream sensor on the exhaust pipe behind the catalytic converter. Clear away any debris.
3. Disconnect the Electrical Connector. Unclip the sensor’s wiring connector. It is usually located up on the chassis or transmission for protection.
4. Apply Penetrating Oil. Spray the base of the sensor where it threads into the bung generously. Let it soak for 10-15 minutes.
5. Remove the Old Sensor. Use your O2 sensor socket and a breaker bar to loosen the sensor. Turn it counter-clockwise. It may require significant force. Be careful not to round the edges.
6. Install the New Sensor. Apply a small amount of anti-seize compound to the threads of the new sensor only (most come with it pre-applied). Do not get any on the sensor tip. Thread it in by hand to avoid cross-threading.
7. Tighten the Sensor. Use the socket and a torque wrench to tighten to specification (usually about 33 ft-lbs). If you don’t have a torque wrench, tighten it snugly, but avoid over-tightening.
8. Reconnect the Wiring. Plug the new sensor’s connector into the vehicle’s harness until it clicks.
9. Clear the Code and Test Drive. Lower the car, start the engine, and use your scanner to clear the P0139 code. Take a test drive to ensure the light does not return and that the monitors reset.
Preventative Maintenance Tips
To avoid future O2 sensor and exhaust issues:
* Use Top Tier fuel to minimize contaminant buildup.
* Address any engine performance problems (misfires, rich/lean conditions) immediately, as they can damage O2 sensors and the catalytic converter.
* Have your exhaust system inspected periodically for rust or damage, especially in areas that use road salt.
* Stick to your Honda’s recommended maintenance schedule for tune-ups.
When to See a Professional Mechanic
Consider taking your Accord to a shop if:
* You are not comfortable working under a vehicle or with electrical diagnostics.
* The diagnosis points to a wiring issue deep in the harness or a potential PCM problem.
* You have confirmed an exhaust leak that requires welding or complex parts replacement.
* The problem persists after replacing the O2 sensor, indicating a deeper issue with the catalytic converter or fuel system.
Frequently Asked Questions (FAQ)
Q: Can I drive my Honda Accord with a P0139 code?
A: Yes, you can usually drive for a short time, but you should adress it soon. Long-term driving can hurt fuel economy and potentially damage the catalytic converter.
Q: How much does it cost to fix a P0139 code?
A: If it’s just the sensor, parts are $80-$200, and labor at a shop adds $75-$150. If the catalytic converter is bad, repairs can cost $1,000 or more.
Q: Is the P0139 code serious?
A: It’s moderately serious. It won’t strand you, but it will cause inefficiency and lead to an emissions test failure. Ignoring it can lead to more expensive repairs.
Q: What is the difference between upstream and downstream O2 sensors?
A: The upstream sensor (Sensor 1) helps the PCM adjust the air/fuel mixture. The downstream sensor (Sensor 2) monitors the catalytic converter’s efficiency. They are often not interchangeable.
Q: Can a dirty air filter cause a P0139?
A: Not directly. However, any condition that severely disrupts the engine’s air/fuel ratio over a long period can stress the entire exhaust system, including the O2 sensors.
Dealing with a P0139 code in your Honda Accord is very manageable with a methodical approach. Start with the simple visual checks and use live data to confirm your diagnosis before replacing parts. In most cases, replacing the downstream oxygen sensor resolves the issue, restoring your Accord’s fuel efficiency and turning off that check engine light for good. Remember, timely attention to engine codes is key to preventing minor problems from becoming major expenses.