Replace Air-Fuel Ratio Sensor: Step-By-Step Guide For Your Car

The air-fuel ratio sensor is an essential component in modern combustion engines, monitoring the amount of air to fuel in the internal combustion engine and providing feedback to the Engine Control Module (ECM) to adjust the fuel injection for optimal combustion. An average air-fuel ratio sensor lasts three to five years, and without it, your vehicle may experience lower fuel economy, higher emissions, and damage to other components. In this article, we will discuss the steps involved in replacing the air-fuel ratio sensor, including the tools required, the workspace needed, and the process of clearing the CEL afterwards.

Average lifespan and how to tell if it's faulty

The air-fuel ratio sensor is an important component in your car's emissions control system. It measures the amount of oxygen in your exhaust gas and sends that information to your car's central computer (ECU). This, in turn, tells your engine whether to optimise for fuel economy or performance.

On average, an air-fuel ratio sensor will last between three to five years. However, this depends on regular maintenance being carried out.

Signs of a Faulty Air-Fuel Ratio Sensor

• Poor fuel economy: A decrease in fuel economy could be a sign of a faulty air-fuel ratio sensor, but it can also be indicative of other problems.
• Rough idle: If your engine is going up and down in RPM, sputtering, and then returning to normal, you may have a rough idle. This can be felt through the floorboard when the car is in "park".
• Engine pinging: A metallic, rattling sound when accelerating from a stop could indicate a failing air-fuel sensor. This is sometimes called a "spark knock" and is caused by an improper combustion cycle.
• Poor gas mileage: If you notice a significant decrease in gas mileage, it could be a sign that your air-fuel ratio sensor needs replacing.
• Increased exhaust emissions: If your car is smoking a lot on startup, or there's a noticeable change in smell or smoke from your exhaust, it could be a sign of a faulty sensor.
• Drop in engine power: If you experience a sudden drop in engine power, this could be a sign of a bad air-fuel sensor.

Differences between oxygen and air fuel ratio sensors

Oxygen sensors and air-fuel ratio sensors are different. An oxygen sensor bounces between rich and lean during normal operation, and the car responds by adding or removing fuel. An air-fuel ratio sensor, on the other hand, calculates the precise mixture and tells the car exactly how much fuel to add or subtract.

Air-fuel ratio sensors can read a much wider range of fuel mixtures than conventional oxygen sensors. They are also called "wideband" oxygen sensors. Unlike oxygen sensors, air-fuel ratio sensors do not produce a voltage signal that suddenly changes when the air-fuel mixture goes rich or lean. Instead, they produce a changing current signal that varies in direct proportion to the amount of unburnt oxygen in the exhaust.

Most cars have at least two air-fuel ratio sensors. There will be at least one in the exhaust system, one in front of the catalytic converter, and one downstream from the catalytic converter.

Location of the sensor

The location of the air fuel ratio sensor varies depending on the car model and make. However, there are some common locations where the sensor can typically be found. Firstly, it is important to note that almost all cars have at least two air-fuel ratio sensors. There is usually one located in the exhaust system, specifically in the exhaust manifold or the front exhaust pipe, before the catalytic converter. This sensor is often referred to as the "upstream" or "front O2 sensor".

Additionally, there is often another sensor located downstream from the catalytic converter, in the rear exhaust pipe after the converter. This sensor is typically called the "downstream" or "rear O2 sensor". The purpose of this rear sensor is to monitor the efficiency of the catalytic converter by measuring the oxygen content in the outgoing exhaust gases.

In some car models, the air fuel ratio sensor may be located in the engine bay, under the hood. It is usually situated near the engine, where it can effectively monitor the air-fuel mixture.

For specific car models, such as the 1990-1999 Mitsubishi Eclipse, Eagle Talon, Plymouth Laser, and Galant VR-4, the optimum location for the sensor is recommended by the manufacturer. In this case, AEM recommends placing the sensor 36 inches away from the turbo outlet, which is typically in the downpipe or the test pipe.

It is important to refer to the specific repair manual or online resources for your car model and make to determine the exact location of the air-fuel ratio sensor.

Tools required

To replace an air fuel ratio sensor, you will need a range of tools and materials. Firstly, ensure you have a nine-volt battery saver, which you can plug into your cigarette lighter to keep your computer live and settings current. You will also need a floor jack to lift your vehicle, as well as jack stands to support it.

For the removal of the old air fuel ratio sensor, you will need a ratchet with metric and standard sockets, a breaker bar, and an air fuel ratio sensor socket. If the sensor is stuck, you may also need a small portable torch to heat up the area around the mounting surface. Ensure you have eye protection and gloves for this step. You will also need an electrical contact cleaner and lint-free cloths to clean the harness and contacts.

When installing the new sensor, you will need a metric thread pitch gauge to measure the thread pitch if you are unsure of the specifications. If you had to remove the exhaust, you will need new exhaust bolts, as the old ones will be weak and brittle. If you had to remove a drive shaft, you will need to tighten the bolts to factory settings. If they are torque-to-yield bolts, they will need to be replaced. If there is an engine guard, you will need a fastener remover and new plastic tabs to reinstall it.

Workspace and preparation

Before attempting to replace the air-fuel ratio sensor, it is important to prepare your workspace and gather the necessary tools and materials. Here are the steps you should follow:

• Park your vehicle on a flat, hard surface. Ensure that the transmission is in park for automatic vehicles or in first gear for manual vehicles. This step is crucial for vehicles with all-wheel drive or rear-wheel drive transmissions.
• Place wheel chocks around the rear tires to prevent any unintended movement. Engage the parking brake to further secure the vehicle.
• Install a nine-volt battery saver into your cigarette lighter to keep your computer live and maintain your current settings. This step is optional, but it can be helpful.
• Open the vehicle's hood and disconnect the battery. Remove the ground cable from the battery's negative post to disable the power to the air-fuel ratio sensor. If you own a hybrid vehicle, refer to the owner's manual for specific instructions on disconnecting the battery.
• Using a floor jack, lift the vehicle from the specified jacking points until the wheels are completely off the ground. Refer to your vehicle's owner's manual for the exact jacking points.
• Place jack stands under the jacking locations and then lower the vehicle onto them. For most modern cars, the jacking points will be on the pinch weld just under the doors along the bottom of the car.

At this point, you have safely raised your vehicle and can proceed to the next steps of replacing the air-fuel ratio sensor. Remember to work cautiously and refer to repair manuals or seek professional assistance if needed.

Frequently asked questions