How to Confirm That Your Toyota's Got a Failing Oxygen Sensor

Like all sensors on a vehicle, oxygen sensors wear out over time. Oxygen sensors are located in the exhaust system. They’re constantly exposed to extremely high temperatures and combustion byproducts. So, they’re more prone to premature failure.

Having one of your oxygen sensors fail is a pretty big deal. The oxygen sensor is responsible for detecting any imbalances in the air/fuel mixture. It then relays the information to the ECU so it can adjust the mixture accordingly. (Read more about the process here.)

OEM O2 sensor

With a bad oxygen sensor, the ECU won’t receive accurate information. Therefore it won’t adjust the air/fuel mixture to the correct ratio. As a result, your engine’s performance suffers in a variety of ways.

The Most Common Symptoms of a Failed Oxygen Sensor

The symptoms start to crop up after the air/fuel ratio gets thrown off. Keep an eye out for the most common symptoms of a rich or lean mixture:

  • Rough, uneven idle.
  • Trouble starting the engine.
  • Reduced fuel economy.
  • Sluggish or lurching acceleration.
  • Strong smell of gasoline when engine is idling.
  • Soot coating the inside of the tailpipe(s).
  • Engine running hotter than normal.

Diagnosing a Failed Oxygen Sensor

A code reader (also called a scan tool or OBDII reader) only tells you that the oxygen sensor is sending an improper reading. It does not tell you if the sensor is bad or good. For some vehicles, it will tell you which sensor is sending the bad signal, which can save you some time.

O2 codeImage Credit: Nathans DIY World

The easiest DIY way to determine whether one of your oxygen sensors is broken is to test all of your oxygen sensors with a digital voltmeter. Checking your oxygen sensors is an easy process that takes about 10 to 30 minutes.

NOTE: There may be between one and four oxygen sensors located throughout your exhaust system. The number varies depending on your vehicle and engine. Refer to your owner’s manual to find out how many your vehicle has and where to locate each one. If you have a code reader, just test the sensor sending the bad signal.

Checking the Voltage Signals

  1. Idle the engine for up to 20 minutes (or drive the car around for the same amount of time).
  2. Put on a thick long-sleeved shirt and thick gloves to avoid burning yourself on any of the exhaust system components.
  3. Optional: lift your vehicle with jack stands or a car lift designed for home use (like a Kwik-Lift, for example).
  4. Set your voltmeter to the millivolt DC scale.
  5. Connect your voltmeter to the oxygen sensor and read the voltage signals. A good oxygen sensor fluctuates between 100mV and 900mV. Anything above or below that range is an indicator of a bad oxygen sensor. The sensor may show a reading that is within the normal range, but stuck in one place. If this happens, either the sensor is bad, or you have another engine problem. To evaluate the sensor, continue to the next steps.

Continue the diagnostic process if your voltmeter reads below 100mV or above 900mV by taking the steps below.

Making the Engine Run Too Lean

You want to make sure the oxygen sensor can detect an imbalance in the air/fuel mixture. You can do this by adding more air to the mixture:

  1. Disconnect the hose between the PCV valve and the intake manifold.
  2. Turn the engine on and then check the reading on the voltmeter. It should quickly read close to 200mV. If the reading is higher, or the sensor reacts slowly you’ve got a broken oxygen sensor. It needs to be replaced as soon as possible.
  3. Repeat for each sensor.
  4. Turn the engine off and then put the hose back where it was before.

If the oxygen sensor provides an accurate reading during this process, the next step is to see how it reacts to an engine that runs too rich.

Making the Engine Run Too Rich

  1. Locate the air cleaning assembly and then disconnect the plastic duct from it.
  2. Partially block the air filter with a rag. (Important - Leave the air filter in place. Block the front of the air filter. If rag were to be placed behind the filter, it will get sucked into the intake.)
  3. Turn the engine on and then check the reading on the voltmeter. The reading should be close to 800mV. If the reading either changed slowly or hasn’t increased as much as it should, then the sensor needs to be replaced.
  4. Repeat for each sensor.
  5. Turn the engine off and reconnect the air duct.

If you find that one of your oxygen sensors needs to be replaced, you can easily do it at home. We will shortly post a comprehensive step-by-step tutorial. Stay tuned!

Written by Jason Lancaster