We had a regular customer stop by the house with a check engine light on. I whipped out my Innova 3100i that I keep at home and pulled a code P0131.
Unfortunately, this is a pretty common code that stands for oxygen sensor voltage low. Often people want to try replacing the sensor, but this is just one reason the code sets in the electronic control module.
I would say only about 50 percent of the time, replacing the part solves the problem. And on these vehicles, the majority of the odometers show mileage well past the 100k range
This article covers the quick story of diagnosis and repair of this code and includes some screenshots from the diagnostic tree chart found in the professional auto repair manual available on the youfixcars.com website. My customer owns a 2008 Nissan Altima v6 with a 3.5 L engine.
The generic code P0131 can set on any car, truck or sport utility vehicle. A quick sidebar about this v6 powered Altima in the story. Most of these mid-sized cars carry the four-cylinder engine with a 2.5 L displacement.
However, this gentleman special ordered one with the V-6 that you find in the older 350 Z sports cars. Anyone considering purchasing a Nissan Altima should drive one with the V-6 before they settle for the four-cylinder model. The difference 2 cylinders makes on this light weight car will shock and awe you.
Diagnosing O2 Sensor Problems
When you pull up a diagnostic tree chart for code P0131 in a professional PDF auto service manual or a similar reference guide you might get scared off. The first impression for this code and the complicated looking steps to resolve it remain daunting.
Fortunately, If you hang in there and start to go through the steps it’s not as bad as it looks. One thing that surprised me about the Nissan service manual is how it refers to the sensor as an AF sensor. This stands for air fuel ratio.
Although I get it, because this component is actually measuring the air fuel ratio and making adjustments to it I’m just from the old school where we called it an O2 sensor because it measure the amount of oxygen in the exhaust stream. Unfortunately, another thing to get past was in the manual they talked about using the specialized diagnostic tool that Nissan technicians use at the dealership level.
Using Simple Tools to Diagnose O2 Sensors
I accomplished the same goals and retrieve the needed information with my extremely simple generic brand automotive scan tool. When diagnosing this code it often boils down into two common problems. Either the sensor failed and isn’t producing voltage or there’s a problem in the wiring harness that supplies power, ground and returns reference signals to the electronic control module.
The conditions that set the code P0131 refer to consistently low voltages or sensors that remain fixed at 0 V. As we discussed in previous oxygen sensor diagnosis articles the voltage that’s monitored by the PCM (Powertrain Control Module) should fluctuate rapidly.
However, at the high and low points of these changes it reaches a maximum of 900 mV and a minimum of about 100 mV. This exhaust gas sensor never stops moving. Something is wrong if it’s fixed at a particular voltage .
And it should never read 0 V. A quick test of the voltage on terminal four of the harness side should read battery voltage. I checked this with a $25 meter that I’ve had around for a decade or more. We had 0 V at that terminal.
Discovering What Happened to this Car
As we all know, sometimes troubleshooting car problems is like solving a mystery. nevertheless, I had 0 V going to the oxygen sensor and a diagnostic tree chart indicates that the most likely cause becomes an open in harness.
As I looked at the underside of the vehicle I could see this car had a story to tell. The car looked clean and shiny from the outside, but the undercarriage of the vehicle was packed with mud and dirt. In addition, there were dents and signs of damage to the floorboards. After showing the customer, he told me that his son borrowed the car the day before.
The very next morning the check engine light came on. After careful interrogation of the young driver he told the true story. He took the 2008 Nissan Altima out into the woods to do some four wheeling. He said, “it kept up with his friend’s bronco and the trails weren’t too harsh”.
One of the possible causes of the P0131 code is an open in the harness that runs to the sensor. This is exactly what happened to this 2008 Nissan. Something jumped up from the trail and literally cut the harness.
In closing, I have two things to mention. When you’re repairing a harness to an oxygen sensor circuit, you really don’t want to use quick crimp connectors. Unfortunately, this circuit operates on millivolts and some resistance can develop in a crimp connector as time goes on. Of course,the parts store would be happy to sell you a new sensor.
We posted a picture at the top of this article representing one that fits the Nissan product line. However, what I wanted readers to get from this story is when you set an oxygen sensor voltage low code, this doesn’t mean the part needs replacing automatically.
Nevertheless, a dead sensor can cause this problem, but it needs logical diagnosis. Please like and share this page with a friend. Also visit the main check engine light page for more articles about specific codes. Visit the YouFixCars.com homepage to find out what else we cover.