What Does An Oxygen Sensor Do? (1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997 Ford Bronco, F150, F250, F350)

The oxygen (O2) sensor, on your F-series pickup, is a key component of the engine management system. It not only helps with fuel economy, it also helps to make sure the engine pollutes less.

In this tutorial I will answer some of the most commonly asked questions about the oxygen sensor.

APPLIES TO: This tutorial applies to the following vehicles:

  1. Ford Bronco: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996.
  2. Ford F150: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996.
  3. Ford F250: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997.
  4. Ford F350: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997.

How Many Oxygen Sensors Does My Pickup Have?

If your F-series pickup is a 1995 or older pickup, then it is OBD I equipped and it will only have one oxygen sensor.

Now, if your Ford pickup is a 1996 model, then it's OBD II equipped and it'll have two or more oxygen sensors.

What Does An Oxygen Sensor Do?

Simply put, the oxygen sensor has the job of monitoring (and reporting) the oxygen content of the exhaust stream that it is connected to.

Depending on the amount of oxygen in the exhaust stream that it's monitoring, it will produce a variable voltage between 0.1 to 1 Volts DC.

Here are some more specific details:

  • If the fuel injection computer injects too much fuel, the exhaust stream will have a low oxygen content.
    • The low oxygen content of the exhasut stream will cause the O2 sensor to output a voltage between 0.5 to 1 Volt DC.
  • If the fuel injection computer does not inject enough fuel, the exhaust stream will have a high oxygen content.
    • The high oxygen content of the exhaust stream will cause the O2 sensor to output a voltage between 0.1 to 0.5 Volts DC.

As you're probably already aware, the oxygen sensor comes equipped with an internal heater.

This heater ensures that the O2 sensor reaches its operating temperature (600°F) as soon as possible and that this temperature can be maintained across all engine operating conditions.

In case you're wondering, the exhaust gas stream that the O2 sensor is connected to, doesn't always reach or maintain the temperature that it needs to operate.

What Happens When An Oxygen Sensor Goes Bad?

Oxygen sensors usually fail in one of three ways:

  • The O2 sensor stops reacting to the oxygen levels in the exhaust stream it is connected to.
  • The O2 sensor responds to slowly to the oxygen levels in the exhaust stream it is connected to.
  • The O2 sensor heater fails and the O2 sensor's "on" time suffers.

In the next section I'll go into some of the specific symptoms you'll see when the oxygen sensor fails.

What Problems Can An Oxygen Sensor Cause?

A bad O2 sensor will cause one or more of the following symptoms:

  • Bad gas mileage.
  • On OBD I equipped pickups, the check engine light is illuminated by one of the following trouble codes:
    • 136: System Indicates Lean (Bank 2)
    • 137: System Indicates Rich (Bank 2)
    • 171: Fuel System At Adaptive Limits -O2 Sensor Not Switching (Bank 1)
    • 172: System Indicates Lean (Bank 1)
    • 173: System Indicates Rich (Bank 1)
    • 175: Fuel System At Adaptive Limits -O2 Sensor Not Switching (Bank 2)

How Can You Tell If An Oxygen Sensor Is Bad?

The surest way to find out if the oxygen sensor has failed, is to test it.

See the next section to find out more.

How Can I Find Out If The Oxygen Sensor Is Bad?

Testing the oxygen sensor is the best way to find out if it's bad or not.

The oxygen sensor diagnostic test involves the following:

  • Checking for O2 sensor trouble codes with a code reader or scan tool.
  • Making sure that the oxygen sensor's heater is receiving 12 Volts.
  • Making sure that the oxygen sensor's heater is receiving Ground.
  • Confirming that the O2 sensor's voltage signal increases as the air/fuel mixture is made richer.
  • Confirming that the O2 sensor's voltage signal decreases as the air/fuel mixture is made leaner.

If the oxygen signal does not increase/decrease as the air/fuel mixture is made richer/leaner, then you can conclude that your F150 (F250, F350)'s O2 sensor is bad and needs to be replaced.

Can I Drive My Pickup With A Bad Oxygen Sensor?

Since the oxygen sensor only helps control fuel economy and engine pollution levels, you're usually able to safely drive the vehicle.

Still, you should test and replace the oxygen sensor as soon as possible.

More Ford F150, F250, F350 Tutorials

You can find more Ford F150, F250, and F350 tutorials in the following index:

  1. Ford 4.9L, 5.0L, 5.8L Index Of Articles.

Here's a sample of the tutorials you'll find in the index:

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Ford Vehicles:

  • Bronco 4.9L, 5.0L, 5.8L
    • 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996
  • F150 4.9L, 5.0L, 5.8L
    • 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996
  • F250 4.9L, 5.0L, 5.8L
    • 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996
  • F350 4.9L, 5.8L
    • 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997