What Does The Oxygen Sensor Do? (Ford E150, E250, E350)

The oxygen (O2) sensor is an integral part of the engine management system of your Ford E-Series van. The fuel injection computer uses it to make sure that the engine runs optimally and that it pollutes less.

In this article I'll answer some of the most commonly asked questions about the oxygen sensor.

APPLIES TO: This tutorial applies to the following vehicles:

• Ford E150: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996.
• Ford E250: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996.
• Ford E350: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996.

How Many Oxygen Sensors Does My Van Have?

All 1987-1995 E-Series vans, which are all OBD I equipped, come equipped with only one oxygen sensor.

Starting with the 1996 model year, all E-Series vans are OBD II equipped and have two or more oxygen sensors.

What Does An Oxygen Sensor Do?

The oxygen sensor is a feedback sensor that helps the fuel injection computer determine if it has injected too much fuel or not enough fuel (as the engine runs of course)

The oxygen sensor does this by reacting to the levels of oxygen in the exhaust stream it's monitoring and producing a correlating voltage signal.

In a nutshell, the O2 sensor works in like this:

• If the fuel injection computer injects too much fuel, the low oxygen content of the exhaust 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 high oxygen content of the exhaust stream will cause the O2 sensor to output a voltage between 0.1 to 0.5 Volts DC.

The oxygen sensor also comes equipped with a heater. Why? Because the oxygen sensor has to reach a certain temperature (around 600°F) to start monitoring the oxygen content of the exhaust.

Since the exhaust gases don't always reach the temperature required by the O2 sensor, the heater makes sure that it does. This ensures that the oxygen sensor can start working almost immediately after the engine starts and in all engine operating conditions (idle, coasting, etc.).

With the O2 sensor's continuous feedback, the fuel injection computer can fine-tune the amount that is necessary to keep the engine running optimally and polluting less.

What Happens When An Oxygen Sensor Goes Bad?

When an oxygen sensor fails, it will stop reporting the amount of oxygen in the exhaust stream it is connected to.

Without the information from the oxygen sensor, the fuel injection computer will not be able to fine-tune the amount of fuel it's injecting into that specific bank of cylinders the oxygen sensor is monitoring.

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 vans, 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 is bad is by testing 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 E150 (E250, E350)'s O2 sensor is bad and needs to be replaced.

Can I Drive My Van 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 E150, E250, and E350 Tutorials

You can find a complete list of tutorials for the full-size Ford E-Series vans here: Ford 4.9L, 5.0L, 5.9L Index Of Articles.

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

• Manifold Absolute Pressure MAP Sensor Test (Ford 4.9L, 5.0L, 5.8L).
• Ignition Coil Test -No Spark No Start Tests (Ford 4.9L, 5.0L, 5.8L).
• How To Troubleshoot A No Start (Ford 4.9L, 5.0L, 5.8L).
• Testing A Blown Head Gasket (Ford 4.9L, 5.0L, 5.8L).
• How To Test Engine Compression (4.9L, 5.0L, 5.8L).