Why O2 Sensor Codes Show Up in Diagnostic Tests

Dashboard Lights - Check Engine Light On

The engine’s fuel injection computer uses oxygen sensors to constantly fine-tune the amount of fuel injected into your engine. Your car will have at least two sensors, and as many as four (unless it’s more than 20 years or so old, in which case it may have only one or none at all). There’s one sensor right ahead of the catalytic converter. This sensor allows the engine computer to constantly adjust the amount of fuel sent to your engine, assuring proper combustion and the best performance and emissions. A second sensor is screwed into the exhaust pipe right past the catalytic converter. This sensor checks the performance of the catalytic converter, making sure it scrubs the last vestiges of pollution from your exhaust.

Here are the most common reasons O2 sensor codes may show up when your check engine light codes are scanned:

  • Obviously, there could be a bad sensor. Sensors can and do fail, usually just by getting lazier and lazier until the computer doesn’t trust their output anymore. When the computer is slewing the air-fuel ratio up and down multiple times per second and the sensor can’t keep up, there’s a code set, illuminating your check engine light. Occasionally, a sensor can fail outright, often by being poisoned by leaded gasoline (not very common nowadays), or atmospheric chemicals. It’s a bad idea to simply replace a sensor without further checking to see if there’s something wrong that made it fail.
  • Bad wiring. Sensors have four fragile wires leading up to them, two for the signal to the computer, and two for a small heating element that helps them get up to their operating temperature more rapidly on a cold startup. Wires that are broken, melted against the hot exhaust pipes or corroded will give erratic or missing readings.
  • A bad spark plug, plug wire or partially plugged fuel injector can cause one or more cylinders to misfire. Because the oxygen in that cylinder isn’t burned up, the extra oxygen in that cylinder winds up passing over the O2 sensor, making the computer think it’s not injecting enough fuel. The danger is that the extra fuel injected to compensate eventually winds up being burned in the catalytic converter, rapidly damaging it. Similarly, a partially plugged fuel injector (there’s one for every cylinder in your engine) might inject too little fuel into a single cylinder, leaving the computer confused about the readings. This will set a code and turn on that pesky CHECK ENGINE light as well.
  • Leaky exhaust pipes. A leaky exhaust pipe obviously can let exhaust gases out of the pipe at the leak. But Air can also be sucked into the pipe at the same leak, and if that leak is upstream of the sensor, it will see the extra oxygen and set a trouble code.
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These are the common things; there are plenty of other, often obscure reasons why an O2 sensor codes might be set.

A trouble code that points to an oxygen sensor is only the first step in your mechanic’s diagnosis of the problem. Most of the issues that turn on your check engine light and set an O2 sensor codes are not a result of a bad sensor. Automatically screwing in a new sensor because there’s a sensor-related code is a big gamble. A good mechanic will always use those trouble codes simply as a starting point in his diagnosis.

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