O2 Sensor Replacement: How to Tell if You’ve Got a Bad Oxygen Sensor

Car repair is an essential part of owning a car. Whether it’s simple tasks like an oil change or bleeding your own brakes, there are plenty of tasks that are DIY friendly and can save you the costs of taking your vehicle to a mechanic. However, it’s essential to ensure that you’ve got the right issue nailed down on more daunting tasks. Diagnostics can be a bit troublesome. So, if you’re thinking about an O2 sensor replacement, it’s best to ensure that is actually your problem.

A bad O2 sensor can cause all sorts of problems

A Bosch LSU4.9 Wideband O2 Oxygen Sesnor in the downpipe of a custom built turbo Volvo 740
Bosch O2 Sensor | Braden Carlson, Motorbiscuit

Cars.com outlines the telltale signs of a bad O2 sensor in a recent article. Though O2 sensor replacement is not a regular maintenance task, most cars will eventually suffer an oxygen sensor failure. However, it’s not always black and white as to whether or not replacing the sensor will alleviate the problem.

For one, many modern vehicles have multiple sensors. OBD-II cars (1996 model years and newer) require a pre-catalytic converter sensor and a second one after the catalytic converter. This is to measure the remnant fuel to determine whether or not the cat is doing its job. V-shaped engines like V6s and V8s typically have four O2 sensors, given their design has two separate exhaust manifolds, meaning most have two catalytic converters.

If you’re lucky enough for a check engine light to come on and specifically tell you which sensor needs replacing, thank your lucky stars. Often, this won’t be the case.

One of the most common check engine light codes that is associated with a faulty O2 sensor is P0420, usually displayed with the message “catalyst system efficiency below threshold.” Typically, this means that the O2 sensor is not reading the air-to-fuel ratio correctly, and it thinks that the catalytic converter has failed. Do keep in mind, though, that there is a possibility that the catalytic converter has failed, too.

Other symptoms of a faulty O2 sensor to look for include rough idling, engine misfires, and a substantial increase in fuel consumption. A bad O2 sensor can cause up to a 40 percent decrease in fuel economy.

A check engine light associated with a bad O2 sensor may also read as a general misfire.

How much does it cost for an O2 sensor replacement?

O2 sensor replacement is a great first dive into more involved car repair. However, some sensors, depending on the vehicle, can be quite a pain to get to. Typically, it takes only a few minutes, assuming you have easy access to the sensor. However, they can often be challenging to remove. The constant expansion and contraction of the exhaust and sensor threads from heating and cooling can get them quite stuck.

Also, if you’re going to attempt the repair yourself, remember that it’s an exhaust component, and it will be extremely hot. Let the car cool down sufficiently before attempting to replace O2 sensors.

Alternately, Repair Pal states that it typically costs between $329 and $379 for a mechanic to replace the sensors for you. However, this could vary depending on the ease of access and how many sensors you are replacing. Some oxygen sensors are as little as $50, while others cost north of $300 just for the part.

Overall, if you’ve determined that an O2 sensor replacement is a solution to your problem, it’s up to you to decide how to go about the repair. However, just keep in mind that it’s not always easy to determine an O2 sensor issue. Furthermore, if the check engine light code doesn’t specify, you may just have to replace all of them.

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