Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.
What Happens if You Have a Bad Oxygen Sensor
A bad oxygen sensor (O2 sensor) can increase:
- hydrocarbon emissions,
- carbon monoxide emissions,
- oxides of nitrogen emission,
- fuel consumption,
and upset engine performance!
Replacing Your Oxygen Sensor
In replacing an oxygen sensor, you need to prevent damage to the mounting threads on the exhaust manifold, pipe, or catalytic converter. It's not uncommon for an O2 sensor to seize, making it difficult to remove. If not careful, you'll seriously damage the mounting threads and end up with an expensive repair. But you can learn how to deal with a stuck sensor and safely replace it in about an hour or so on a Saturday morning—right in your own garage.
Here is how.
Preparing to Remove the O2 Sensor
If the sensor is mounted on a flange, follow these steps:
- Unplug the electrical connector.
- Unscrew the mounting bolts.
- Remove the old sensor.
- Install a new unit along with a new gasket. That's it. You're all done.
For all other types of O2 sensors, follow these steps:
- About one or two hours before you plan to remove the sensor, it's a good idea to apply to the sensor’s base—where it joins the exhaust manifold or pipe—a good penetrating oil spray. Do this a couple of times, following the instructions on the product's package.
- After spraying the sensor, start and idle the engine for about 10 minutes. This will warm up the exhaust system and make removing the sensor much easier.
- After that, disconnect the negative battery cable—the black cable hooked to the battery post with the negative sign (-) next to it. Isolate the disconnected terminal by wrapping a rag around it to prevent it from accidentally touching the battery post.
- When you need to replace a sensor close to the catalytic converter, you'll probably have to lift the front of your vehicle. If so, park your vehicle on a level surface. Use a floor jack to raise it and support your car on a couple of jack stands. Block the rear wheels with wooden blocks and apply the parking brakes. And, whenever you need to crawl under your vehicle to do some work, wear safety glasses to prevent dirt or grease from falling in your eyes.
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Removing the Oxygen Sensor
- Unplug the O2‘s electrical connector carefully to avoid breaking the plastic locking tabs.
- Then, turn the sensor counterclockwise using an open-end wrench or an oxygen sensor socket. This sensor socket has a slit on one side to fit the sensor's wiring harness through during removal or installation.
- If the sensor seems frozen in place or seized, stop. Cut the wiring harness off the sensor and use a six-point deep well socket (or spark plug socket of the correct size, usually a 7/8-inch socket) and a ratchet or breaker bar instead.
- Apply penetrating oil to the base of the sensor, following the instructions on the product's label.
- Try to turn the sensor just a few degrees counterclockwise, just until the sensor refuses to move anymore.
- Apply penetrating oil to the base of the sensor and tighten it again, turning the sensor clockwise.
- Repeat steps 5 and 6 as many times as necessary. Remember not to force the sensor loose. Just turn the unit counterclockwise until the sensor refuses to move. Apply penetrating oil and tighten it again. Gradually, the sensor will begin to come loose.
- Once you've removed the sensor, apply penetrating oil to the boss (mounting) threads in the exhaust manifold, tube or catalytic converter, and clean the threads using a nylon brush.
- Closely check the boss threads. If the threads seem nicked, a bit flattened or have some other minor damage, use a thread chaser to repair the threads. However, if the threads seem crushed or stripped, use a tap to repair them. Check a drill and tap size chart to get the correct tap size for the job.
- After the repair, thoroughly clean up the threads of any metal shavings using penetrating oil and a clean rag.
Installing the Oxygen Sensor
- After removing the old sensor, compare it to the new one; or bring the old sensor when you buy the new one and make sure both match. If possible, try to replace it with an OEM unit instead of a "universal" type. Otherwise, you may need to cut off the electrical connector from the old oxygen sensor and splice it to the new sensor.
- If the new sensor doesn't come with it, apply a dab of anti-seize compound to a small spot on the threads. This will help you remove it when its service schedule comes up again.
- When handling the new sensor, do not touch the tip or contaminate it with oil, grease, dirt or any other kind of foreign matter. Contamination will cut short the new sensor's service life.
- Hand-start the sensor when screwing it in, to prevent cross threading.
- Using an oxygen sensor socket and a torque wrench, tighten the sensor to the torque listed in your service manual. If you don't have the manual, buy an inexpensive, aftermarket repair manual from your local auto parts store or online.
- Plug in the sensor electrical connector.
- Route and secure the wiring away from hot surfaces.
- Now, lower your vehicle, if you had to raise it, and reconnect the battery negative cable.
Do You Need to Replace the Other Oxygen Sensors?
Most modern vehicles use more than one oxygen sensor. When servicing a sensor as part of your vehicle maintenance schedule, you might want to replace the other ones at the same time. Check your vehicle’s service manual to locate all the sensors in your particular car model.
When replacing an oxygen sensor because of a malfunction indicator light (MIL) or Check Engine light, you'll need to erase the diagnostic trouble code (DTC) from the computer memory. On 1996 and newer models, erase the code using a scan tool. If you don't own a scan tool, check with your local auto parts store. They usually do it for you free of charge.
On older models, you need to apply a particular method, depending on your vehicle make and model. Check your service manual for the proper procedure.
Car manufacturers recommend replacing the O2 sensor about every five years (one- and two-wire sensors) or 10 years (three- and four-wire sensors). However, silicon or carbon soot may cover an oxygen sensor due to a car repair or engine performance problems. In this case, you'll need to replace it before its due service schedule. A worn-out or bad oxygen sensor will increase emissions, cause engine performance problems, and step up fuel consumption by up to 40%.
But you just learned how to replace an oxygen sensor without damage to the exhaust manifold, catalytic converter, or exhaust pipe. Save this guide for the next time you need to service the sensor, and save some money and time.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
Question: Is replacing the PROM PCM Chip the same as replacing the ECU?
Answer: The ECU has many input and output circuits that can go wrong. Replacement of an ECU is usually done because of overloads, corrosion, or damaged due to heat or vibration, for example. The PROM only has the programming instructions for the intended application.