How to Diagnose a Failing Oxygen Sensor

Every exhaust system has between one and four oxygen sensors. An oxygen sensor is a tube-sized sensor in your vehicle's exhaust system. It keeps tabs on the oxygen in the exhaust fumes in order to ensure that your engine’s fuel injection system is working properly. This article goes more into detail about oxygen sensors and how they work.

Symptoms of a Failing Oxygen Sensor

Many oxygen sensors will last more than 100K miles. But they all eventually wear out. They fail because they’re exposed to high temperatures and byproducts of combustion. Oxygen sensors measure the oxygen in the exhaust and compare it to the oxygen in the air surrounding the vehicle. The ECU uses this information to adjust the air/fuel mixture. When an oxygen sensor fails, the ECU won’t receive the information. Therefore it won’t adjust the air/fuel mixture accordingly.

Here are the biggest telltale signs that you need to replace one (or more) of your oxygen sensors:

  • Rough idle
  • Poor fuel economy
  • Failed emissions testing
  • Trouble starting the engine
  • Illumination of the check engine light

If you have a hunch that one of your oxygen sensors is failing, you can bring your vehicle to a Nissan dealership and have them do a diagnostic check. P0138 and P0135 are the most common codes for oxygen sensor failure. Or you can diagnose the problem yourself using the simple step-by-step tutorial below.

The Diagnostic Process

DTC code p0135

Image Credit: Edmunds

If you have more than one oxygen sensor, all of your oxygen sensors must be tested with a digital voltmeter. Use a 10-megaohm impedance voltmeter. This impedance level prevents the meter from drawing too much current and damaging your oxygen sensor(s). To do this:

  1. Warm up your car by either:
    1. Idling the engine for 15-20 minutes.
    2. Taking it out for a 20-minute spin.
  2. Turn off the engine.
  3. Set your voltmeter to the mV (millivolt) DC scale.
  4. Locate the oxygen sensor(s).
  5. Lift your car safely with a home lift (like a Kwik-Lift) or jack stands.
  6. Connect your voltmeter to the oxygen sensor. Be extremely careful because the exhaust manifold and tailpipes are hot and can burn you.
  7. Turn on the engine and read the oxygen sensor’s voltage signals on your meter. If the reading consistently fluctuates between 100mV and 900mV, then your oxygen sensor is likely okay.
    1. If the reading stays under 100mV or over 900mV, then you either have a broken oxygen sensor or an engine problem. Follow the next steps to determine if the oxygen sensor that needs to be replaced.
  8. Turn off the engine and pop the hood. Then disconnect the hose between the positive crankcase ventilation valve (PCV valve) and the intake manifold. This increases the air in the air/fuel ratio to create a lean mixture.
  9. Turn the engine back on and then check the oxygen sensor’s reading again. If it reads close to 200mV, then the sensor should be working fine.
    1. If the reading doesn’t change (from last time), then your oxygen sensor isn’t working quite as it should.
  10. Now, turn off the engine and put the hose back in place between the PCV valve and the intake manifold.
  11. Remove the plastic duct to expose the air filter, but leave the filter in place. Partially cover the filter with a rag. This will reduce the amount of air the engine gets. The idea is to cause a rich mixture.
  12. Turn the engine back on and then check the oxygen sensor’s reading. If it emits a signal of about 800mV (and if it’s been working okay up to this point), then the oxygen sensor is fine.
    1. If the sensor doesn’t respond appropriately, then it’s broken and needs to be replaced.
  13. Reconnect the air duct and then repeat the process with the rest of the oxygen sensors (if any).

If you find that one of your oxygen sensors is failing, then you need to replace it as soon as possible. The good news is that it’s certainly a project you can do at home. All you need are a few basic tools and this comprehensive tutorial.