P0137 Diagnostic Tests -Rear O2 Sensor (2003-2007 2.4L Honda Accord)

A P0137 diagnostic trouble code (DTC) usually points to the rear oxygen sensor on your 2.4L Honda Accord. This is the downstream sensor (O2 Sensor 2, Bank 1) that monitors catalytic converter efficiency and reports back to the PCM.

In this tutorial, I'll explain why this code sets, and I'll walk you step by step through how to check the rear O2 sensor's output to confirm whether it's really fried or not.

The good news? You don't need a factory scan tool or dealership-level diagnostic equipment. A generic scan tool with live data capability, a multimeter, and some basic know-how will get the job done.

NOTE: The rear O2 sensor goes by a few different names —downstream O2 sensor, post-catalytic converter O2 sensor, oxygen sensor bank 1 sensor 2 (O2S12) —but they all refer to the same sensor.

APPLIES TO: This tutorial applies to the following vehicles:

• 2.4L Honda Accord: 2003, 2004, 2005, 2006, 2007.

02 SENSOR CIRCUIT WIRING DIAGRAM:

• O2 Sensors Circuit Wiring Diagram (2003-2007 2.4L Honda Accord).

REAR O2 SENSOR DIAGNOSTICS:

• P0137 Diagnostic Tests -Rear O2 Sensor (2003-2007 2.4L Honda Accord).
• P0141 Diagnostic -Rear O2 Sensor Heater Tests (2003-2007 2.4L Honda Accord).
• Rear Oxygen Sensor Performance Tests (2003-2007 2.4L Honda Accord).
• P0420 Diagnostic -Catalytic Converter Tests (2003-2007 2.4L Honda Accord).

What Does A P0137 DTC Mean?

Code P0137: O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2) means the PCM has detected that the rear oxygen sensor's output signal is stuck low —specifically at or below 0.29 Volts— for longer than it should be.

To really make sense of this (and understand why this is a gold mine of info when it comes to diagnosing the rear O2 sensor as bad), we need to know that under normal operation:

• The rear O2 sensor produces a voltage that swings slowly between about 0.1 and 0.9 Volts.
• Its sole purpose is to monitor the catalytic converter's performance and report on its efficiency to the PCM.

We also need to keep in mind (and this is key):

• Every gasoline engine lets a little unburned fuel slip past —complete combustion just isn't possible, so hydrocarbons will always show up in the exhaust.
• The PCM leans on the front O2 sensor (air/fuel ratio sensor) to fine-tune the engine's air/fuel mixture, but these adjustments are never perfect. Sometimes it overshoots by adding too much fuel, other times it cuts back more than it should.
• When extra fuel gets through, the catalytic converter handles most of it, but it can't eliminate every single trace of unburned hydrocarbons.
• This is why the rear O2 sensor voltage never sits flatlined high or low. Instead, it drifts back and forth, slowly moving between a lean signal (about 0.1 V) and a rich signal (about 0.9 V).

So, when the PCM expects to see the rear O2 sensor responding to exhaust mixture changes (lean vs rich) but instead finds it flat-lining low, it sets a P0137.

Symptoms Of A P0137 DTC

Most of the time, a P0137 will just trigger the check engine light (CEL) with little to no change in how your 2.4L Honda Accord runs.

The real problem shows up at inspection time. If your county requires a safety/emissions test, that CEL shining nice an bright on the instrument panel and stored P0137 in the PCM's memory will keep your Accord from passing until it's fixed.

Should I Just Replace The Sensor Without Testing It?

For lots of folks, the go-to diagnostic method for a P0137 trouble code is to simply replace the sensor and see what happens. And I'll be honest with you —about 8 times out of 10, it solves the issue.

The things that make this approach attractive are:

1. Replacing the sensor isn't that difficult —it can be done in under 30 minutes.
2. The rear O2 sensor isn't that expensive.
3. Most folks don't have a scan tool to test the sensor's output anyway.

Even if replacing it doesn't solve the issue, well, that's just part of the diagnostic process for them. And since they did it themselves, they're only out the cost of the sensor —no shop labor or diagnostic fees.

I want to give you my own two cents on this diagnostic strategy, and it's that you've got a better chance of it working out if:

• There aren't any other codes stored along with the P0137. If there are, those codes need to be diagnosed and fixed first.
• You've inspected the section of the rear O2 sensor's wiring that runs between the sensor and the body. Make sure it isn't touching the exhaust pipe or catalytic converter. In other words, those wires can't be burned, shorted together, or damaged in any way, shape, or form.

So, if you just want to replace the rear O2 sensor right off the bat, no problem. I've got you covered. Below are my two rear O2 sensor recommendations. They're both from brands I've trusted for years and never hesitate to recommend:

• Walker Products 350-34122 Oxygen Sensor (Amazon affiliate link).
• Denso 234-4797 Oxygen Sensor (Amazon affiliate link).

That being said, I always recommend testing the rear O2 sensor before replacing it. The check doesn't take long, it's simple to do, and all you need is a basic scan tool with "live data" functionality. It's absolutely something you can handle yourself.

And if you're like me, you probably want proof that the sensor is truly bad before dropping money on a replacement. That's exactly what this tutorial is about —I'll guide you step by step through the tests.

Where To Buy The Oxygen Sensor And Save

If it's time to replace the rear O2 sensor, here are my two go-to recommendations. These come from brands I've trusted for years, and I can confidently suggest them to you without hesitation:

TEST 1: Checking Rear O2 Sensor Voltage

The first thing we're gonna do is confirm whether the rear O2 sensor signal is really stuck at or below 0.29 Volts (which is what the P0137 DTC is accusing the rear O2 sensor of doing).

We'll do this using a scan tool with live data capability. If you don't have a scan tool with live data capability, this is the one I recommend:

• ZM301 OBD2 Scanner Diagnostic Tool (Amazon affiliate link).

Alright, this is what we need to do:

1. 1

   Connect your scan tool and go to the live data section.

2. 2

   Locate the PID labeled O2S12 (this is the rear O2 sensor, Bank 1 Sensor 2).
   
   NOTE: Depending on the scan tool, this PID may be labeled differently. Check your scan tool's user manual to see how it appears on your particular model.

3. 3

   Start the engine and hold it at about 2,000 RPM until the radiator fan comes on.

4. 4

   Once the radiator fan comes on, let the engine return to idle.

5. 5

   Watch the O2S12 voltage at idle: Is it stuck at or below 0.29 Volts, or is it slowly swinging between 0.1 and 0.9 Volts?

6. 6

   Snap the throttle a few times up to about 3,000 RPM, then let it settle back to idle.

7. 7

   Check the O2S12 voltage again at idle: Is it still stuck at or below 0.29 Volts, or does it show normal slow swings between 0.1 and 0.9 Volts?

Now, let's interpret your results:

CASE 1: O2S12 voltage stays stuck at or below 0.29 Volts (both at idle and after snapping the throttle). The PCM is seeing a low-voltage condition, which confirms that the P0137 DTC is valid.

The next step is to force a rich condition on the engine (while it's running) and see if the rear O2 sensor reacts to it. For this test, go to: TEST 2: Manually Inducing A Rich Condition.

CASE 2: O2S12 voltage slowly swings between 0.1 and 0.9 Volts (at idle and after snapping the throttle). The sensor is working correctly at this time.

The condition setting the P0137 may be intermittent (and these are always harder to troubleshoot and solve). Erase the code and road-test the vehicle. If the code comes back (it may take a day or two), repeat this test.

TEST 2: Manually Inducing A Rich Condition

If the O2 sensor is alive and kicking, it should respond immediately when we enrich the air/fuel mixture manually.

We'll enrich the air/fuel mixture with a short spray of starting fluid into the intake with the engine idling (which will instantly create a rich exhaust condition the rear O2 sensor should react to).

If your Accord is equipped with a mass airflow (MAF) sensor, you don't want to spray starting fluid directly into the throttle body. Doing that would bypass the MAF completely, and the PCM wouldn't see the airflow change.

On MAF-equipped models, you'll need to loosen the top of the air filter box and spray the starting fluid in through there. This keeps the MAF in the loop and helps us complete the test without any complications.

On Accords without a MAF sensor, you can disconnect the intake duct right at the throttle body and introduce the starting fluid directly.

Here's how the rear O2 sensor should behave:

• Working sensor: The rear O2 should respond immediately, spiking up toward 0.9 Volts as soon as the extra fuel enters the intake.
• Failing sensor: If there's no change in voltage, or the reaction is sluggish, then the rear O2 sensor isn't functioning correctly and needs to be replace to resolve the P0137 DTC.

Here's what we need to do:

1. 1

   Hook up your scan tool and open the live data section. Find the parameter for the rear oxygen sensor —it's usually listed as O2S12 (Bank 1 Sensor 2).

2. 2

   Start the engine and let it reach full operating temperature.

3. 3

   With the engine idling, introduce a quick shot of starting fluid into the intake: spray through the throttle body if the Accord doesn't have a MAF sensor, or through the loosened air filter box lid if it does.
   
   Only a short burst is needed —just enough to enrich the mixture. If you spray too much, the engine will stall.
   
   NOTE: If the engine stalls, you've added too much. Restart it, let it idle for a minute or two, and repeat this step with less spray.

4. 4

   Watch the rear O2 sensor signal on your scan tool as you spray the starting fluid.

5. 5

   A good rear O2 sensor will spike immediately above 0.8 Volts.

6. 6

   When the test is done, shut the engine off and re-secure the air duct or air filter box.

Here's what those test results mean:

CASE 1: O2S12 voltage spiked immediately to around 0.9 Volts when the starting fluid was introduced. This confirms the sensor is alive and able to detect a rich condition at this moment.

Now, even though the rear O2 sensor responded correctly here, in TEST 1 it was stuck showing a continuous lean condition (0.29 Volts or less). This points to the sensor failing intermittently —or another issue being the real culprit. To dig a little deeper, go to: Rear O2 Sensor OK, But P0137 DTC Keeps Coming Back.

CASE 2: O2S12 voltage did not respond —it stayed stuck low during the enrichment test. The rear O2 sensor is not doing its job and needs to be replaced.

When you're ready to replace the rear O2 sensor, below are my two recommendations. They're from trusted brands I've used for years and don't hesitate to recommend:

• Walker Products 350-34122 Oxygen Sensor (Amazon affiliate link).
• Denso 234-4797 Oxygen Sensor (Amazon affiliate link).

Rear O2 Sensor OK, But P0137 DTC Keeps Coming Back

If you've made it this far, then in TEST 2 you confirmed the rear O2 sensor (O2S12) is alive and working. It reacted right away when you enriched the air/fuel mixture with starting fluid, which tells us the sensor itself isn't the issue.

Yet the PCM is still flagging a P0137 DTC, which tells us the rear O2 sensor voltage is spending too much time at or below 0.29 Volts. This means the sensor is just reporting what it's actually seeing —a consistently lean condition in the exhaust.

Here are the most common causes for that lean condition:

• Exhaust leak before the rear O2 sensor: A crack in the exhaust pipe (small or large) or a bad exhaust manifold or exhaust pipe gasket ahead (upstream) of the rear O2 sensor, can let in fresh air. This extra oxygen fools the O2 sensor into reporting a lean condition and cause the PCM to set a P0137 DTC.
• Fuel delivery issues: A weak fuel pump, clogged fuel filter, or dirty injectors can all lean out the mixture. Even with the front O2 sensor and PCM constantly trying to compensate and add more fuel, it may not be enough and the rear O2 may still end up spending most of its time stuck low.
• Vacuum leaks: Unmetered air sneaking into the intake (through cracked hoses, a bad intake gasket, or PCV issues) will drive the mixture lean, which will also affect the rear O2 sensor output.
• Front O2 sensor problems: If the upstream O2 sensor is lazy (responding too slowly), the PCM may pull more fuel than it should. The rear O2 then reflects this lean mixture and sets the code.
• PCM software or internal fault (rare): If everything else checks out, it's possible (though uncommon) that the PCM has a logic or internal failure that causes it to misinterpret the sensor's input.

The thing to keep in mind is that you've confirmed the rear O2 sensor can switch rich when it needs to (TEST 2). If P0137 keeps coming back, then you're looking at an actual lean condition in the exhaust stream —not a bad sensor.

More 2.4L Honda Accord Test Tutorials

You can find a complete list of 2.4L Honda Accord wiring diagrams and diagnostic tutorials in this index:

• 2.4L Honda Accord Index Of Articles.

Here's a sample of the tutorials you'll find there:

• How To Test Misfire Trouble Codes (2003-2006 2.4L Honda Accord).
• How To Test The Engine Compression (2003-2006 2.4L Honda Accord).
• How To Test The Fuel Injectors (2003-2006 2.4L Honda Accord).
• How To Test The Fuel Pump (2003-2007 2.4L Honda Accord).