Testing a P0131 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1) is not hard.
In this tutorial, I'm gonna' show you some of the basics you need to know about what DTC P0131 means and a step-by-step test of the upstream oxygen sensor.
Although this tutorial is geared toward a 3.9L, 5.2L, or 5.9L Dodge/Jeep pick up, van, or SUV, the info and test can be used on any 1996-2003 make or model.
Contents of this tutorial:
P0131 Basics You Need To Know
Before we jump into diagnosing the P0131 Diagnostic Trouble Code (DTC) that's lighting up your check engine light (CEL), in this section I'll share with you some basic, but important information.
LOCATION: The oxygen sensor, that trouble code P0131 is referring to, is located before the catalytic converter and is known by several different names. The most common are:
- Upstream oxygen sensor.
- O2S11.
- Pre-catalytic converter oxygen sensor.
- Front O2 sensor.
Some vehicles have only one upstream oxygen sensor and some have two upstream oxygen sensors. Here are some more specifics:
- On vehicles with two upstream oxygen sensors, a P0131 DTC is identifying the one that's on Bank 1. Bank 1 is the bank of cylinders that have cylinders 1, 3, 5, and 7 (5.2L and 5.9L V-8 engines) and 1, 3, and 5 (3.9L V-6 engines).
- On vehicles with one upstream oxygen sensors, a P0131 DTC is identifying the only one that's before the catalytic converter.
DEFINITION: P0131 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1) diagnostic trouble code (DTC) is telling you:
- That the oxygen sensor, located before the catalytic converter, is reporting a continuous low voltage to the PCM (Powertrain Control Module). This continuous voltage will be a value under 0.5 Volts (500 millivolts).
- This continuous low voltage is being reported by the upstream O2 sensor for more than 2 minutes.
- This continuous low voltage will be a voltage value between 0.1 Volt to 0.5 Volts (100 to 500 millivolts).
- Normally, any continuous voltage, from the upstream O2 sensor, below 0.5 Volts indicates a Lean condition but not always, since the O2 sensor can fail and produce the same result (although a Lean condition does not really exists)
- A Lean condition simply refers to the fact that the engine's air/fuel mixture is too oxygen heavy (in other words, not enough fuel is present for proper/ideal combustion). This is usually due to a large vacuum leak, or a bad fuel pump, etc.
OPERATION: In a normally operating engine, the front (upstream) O2 sensors will produce a voltage that will constantly and rapidly change between 0.1 Volt to 0.9 Volts DC.
So, when a P0131 pops up, the PCM is seeing a voltage that stays continually fixed somewhere below 0.5 Volts for more than 2 minutes.
PROBABLE CAUSES: Several things can cause a P0131 diagnostic trouble code, the most common causes of a P0131 DTC are:
- Bad oxygen sensor.
- This is probably the most common cause of a P0131 DTC.
- The oxygen sensor fails internally in such a way that it stays stuck producing a continuous low voltage. This low voltage will be something below 0.5 Volts (500 millivolts).
- A lean condition due to a:
- Failing fuel pump.
- Dirty fuel filter.
- Clogged fuel injectors.
- Vacuum leak from bad intake manifold gaskets or a vacuum hose(s).
- PCM not getting the correct signal due to a short in the O2 sensor's wiring.
- This usually the end result of the O2 sensor's wiring or the engine wiring harness coming into contact with the exhaust pipe, exhaust manifold or a hard edge (usually on the engine) that causes wiring's insulation starts to melt.
- Bad PCM. This is extremely rare.
With this basic info we can now start troubleshooting the P0131 DTC, let's turn the page and get testing.
How To Diagnose Trouble Code P0131 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)
As mentioned in the previous page, diagnosing a P0131 is not that difficult.
There are 2 important things to do: First, check that the upstream O2 sensor's wiring is not melted and stuck to the exhaust pipe.
NOTE: The engine should be completely cold when checking the oxygen sensor's wiring for shorts and/or to see if it has melted to the exhaust pipe or you run the risk of severe burns. The exhaust pipe, and especially the catalytic converter, stay very hot even after the engine has been shut down.
Second: You need to confirm that the front (upstream) O2 sensor voltage is really stuck below 0.5 Volts with a scan tool that has Live Data capability (Don't have a scan tool? Need a scan tool? check out my recommendation: Actron CP9580 Scan Tool Review).
In the test steps below, I'll show you in a step-by-step manner how to induce a Rich condition with carburetor spray.
This is what you'll need to do:
- 1
Connect your scan tool to the your car or mini-van and start the engine (Don't have a scan tool? Need a scan tool? check out my recommendation: Actron CP9580 Scan Tool Review).
Let the engine idle for at least 15 minutes before you start the test, to get the O2 sensor to activate. - 2
Now, on your scan tool (and once you're in Live Data mode), scroll down to the PID that's labeled O2S11. This PID is the one that will show you what the oxygen sensor is reporting in Volts DC.
- 3
Now, take a look at the voltage readings for O2S11.
These should be constantly moving between any number between 0.100 Volts and 0.900 Volts.
If the voltages are not moving between 0.100 and 0.900 Volts, don't worry about it just yet, let's go on to the next step. - 4
With the engine running, spray a little carburetor cleaner into a vacuum hose (that has engine vacuum) while you observe your scan tool's display screen.
You're not going to be able to spray carb cleaner into the throttle as the engine is running, because if you were to do this, the engine will die as soon as you disconnect the air duct from it (to spray into it).
If you spray too much, the engine will stall. If this happens to you, just restart the engine and repeat the step and spray less carb cleaner spray. - 5
As you spray some short burst of carb cleaner into the vacuum hose, you should see the voltage numbers of O2S11 immediately spike to 0.800 to 0.900 Volts. And as long as you're spraying, these voltage number should stay there.
- 6
When you stop spraying, the O2 sensor values should come down and within a few seconds, they should start oscillating between 0.100 Volts to 0.900 Volts. If they don't, don't worry about it yet.
Let's take a look at what your test results mean:
CASE 1: The upstream O2 sensor's voltage shot up to 0.9 Volts. This tells you that the O2 sensor is operating normally and that the problem causing the P0131 is elsewhere.
You'll need to test to see if the P0131 is being caused by:
- A lean condition due to a:
- Failing fuel pump. This is a fuel pump that starts and keeps the engine running, but can no longer supply the appropriate amount of fuel volume anymore.
- Dirty fuel filter.
- Clogged or inoperative fuel injector(s).
- Vacuum leak from bad intake manifold gaskets or a vacuum hose(s).
CASE 2: The upstream O2 sensor's voltage DID NOT shoot up to 0.9 Volts. This tells you that the upstream oxygen sensor is bad and needs to be replaced.
If the oxygen sensor was operating normally, it would have reacted to the carburetor cleaner immediately by producing a voltage around 0.9 Volts (900 millivolts) since it did not, you can deduce correctly that it's fried and needs to be replaced.
CASE 3: The upstream O2 sensor's wiring is melted to the exhaust pipe. Replace the upstream oxygen sensor.
Oxygen Sensor Codes Keep Coming Back
So you tested the upstream oxygen sensor(s) and they tested good or you have already replace one or both and yet the PCM keeps accusing them as being bad (which also means that the check engine light is back on).
When this happens, it's usually due to either a Rich condition or Lean condition that's being caused by some other component on the engine.
What sucks about this, is that whatever is causing the Rich or Lean condition is not present all of the time (specially when you're conducting the tests). This happens quite a bit. The strategy here is to wait a few days for the condition (that's causing the O2 sensor issues) to get worse. When this occurs, you'll be able to test it and solve it.
Where To Buy The O2 Sensor And Save $$$
I used to buy everything I needed (part-wise) for my vehicle at my local auto parts store, until I realized just how over-priced everything is (sad, since most of what they sell are Chinese knock-offs that should be selling for a whole lot less!).
Buying my parts online has been one of the biggest money saving decisions I've ever made and I think you'll benefit from it too.
At the top, you'll find a link to an oxygen sensor that you can follow. Once you get there, amazon.com will ask you for the specifics of you particular vehicle to make sure it fits. If it doesn't, they'll show you several that will. It's that easy to find the right part!
Give it a try and see for yourself just how much you can save! I think you'll agree that buying it online is a whole cheaper and can be done without having to deal with the hassle of getting to the auto parts store.
More Test Articles
I've written quite a few 3.9L, 5.2L, 5.9L Dodge ‘how to’ tutorials that may help you troubleshoot the issues on your Dodge van, pick up or SUV. You can find all at: 3.9L, 5.2L, 5.9L Dodge Index Of Articles.
Here's a small sample of the articles/tutorials you'll find in the index:
- How To Test A No Start Condition (Dodge 3.9L, 5.2L, 5.9L).
- How To Diagnose Misfire Codes (Dodge 3.9L, 5.2L, 5.9L).
- How To Test The Fuel Injectors (Dodge 3.9L, 5.2L, 5.9L).
- How To Test The Ignition Coil -No Start Tests (Chrysler 3.9L, 5.2L, 5.9L).
- How To Test The MAP Sensor (P0107, P0108) (Dodge 3.9L, 5.2L, 5.9L).
If this info saved the day, buy me a beer!
