Got a p0420 Code? You May Need an Oxygen Sensor. Or You May Need Something Else Entirely.

p0420 code oxygen sensor

When check engine lights are scanned, a p0420 code is a very common result. It’s actually a generic code, meaning it can be thrown from any vehicle after 1996. Commonly, people think the p0420 code indicates that the oxygen sensors need to be replaced, but that’s definitely not always the case.

p0420 Code: Time to Replace an Oxygen Sensor?

Background on Oxygen Sensors

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.

Oxygen sensors are only a part of a greater system, meaning if the p0420 code (or p0141, or p0135) is thrown, that doesn’t necessarily indicate the oxygen sensors need replacement. Rather, there could be something else in that system does that’s causing the oxygen sensors to appear faulty. This is actually the case with most check engine codes.

Common Causes of a p0420 Code (or Something Similar)

  • 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. As you’ll see, it’s a bad idea to simply replace a sensor. It’s wise to check further and 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.

  • Bad Spark Plug, Wire, or Fuel Injector

    Any of these 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. This makes 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. This is rapidly damaging. Similarly, a partially plugged fuel injector (there’s one for every cylinder in your engine) might inject too little fuel into a single cylinder. The computer can become confused about the readings. Any confusion will throw a code and 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. If that leak is upstream of the sensor, it will see the extra oxygen and set a trouble code.

  • Bad Catalytic Converter

    This is a costly repair, but common with a p0420 code. For this reason, we highly recommend a diagnosis prior to performing any replacements.

These are the common things; there are plenty of other, often obscure reasons why an O2 sensor codes might be set.

Now That You Have Some Background, Get to a Mechanic

A trouble code that points to an oxygen sensor (such as p0420, p0135, p0141, or others) is only the first step in your mechanic’s diagnosis of the problem. It turns out that most of the issues that set oxygen sensor codes are not a result of a bad sensor.

So, 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. A good customer that’s done their research (you!) will ask for a diagnosis. Now you know why you shouldn’t just ask for quick replacement of the first part a check engine code points to.

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