How to Test an Oxygen Sensor

(This testing is for 4 wire Heated Oxygen Sensors only and not wide-band or A/F ratio sensors)

What are the symptoms of a bad oxygen sensor?

How to locate the correct oxygen sensor for testing

  • Whether you are diagnosing a trouble code or just testing a sensor for proper operation, the first step is to locate the correct or suspect sensor.
  • To locate the correct o2 sensor, determine if it is a bank 1 (bank with cylinder #1) or bank 2 (bank without cylinder #1) sensor. Then if it is an upstream (located before catalytic converter) or downstream (located after catalytic converter) sensor. If working off a trouble code this information will be found in the DTC description
  • Use the cylinder identification and firing order chart to locate the sensor on the vehicle
Tip: Look closely at exhaust routing, some sensors are located very close to each other, this can make it hard to identify the correct senor location see example below

How to test an oxygen sensor

Visual inspection

Difficulty: 1/5 Time: 10 minutes
Tools required:


Inspection Mirror
  • Visually inspect the oxygen sensor for proper installation
  • Check the O2 sensor wiring and connector for damage and corrosion
Tip: Due to the location of oxygen sensors it is common for them to come in contact leaking fluids, hot exhaust components or road debris. This can result in failure of o2 sensors, wiring and connectors
  • Inspect the wiring harness back to engine control module (ECM) paying close attention to vulnerable areas around hot exhaust components and sharp metal edges. Look for any spots where the wiring may have melted or rubbed through

How to test the oxygen sensor heater

Difficulty: 2/5 Time: 30-60 minutes
Tools required:


  • Locate the oxygen sensor heater wires, 4-wire heated oxygen sensors have 2 wires for the internal heater element.
Tip: The oxygen sensor heater wires are typically the 2 wires that are the same color
  • Using a multimeter set to Ohms, measure the resistance between the two heater wires and record the reading. How to use your Ohm meter
  • Heater resistance is typically around 4-25 ohms, but make sure to check vehicles listed specification
  • If heater resistance is out of specification, replace faulty oxygen sensor

For more detailed o2 sensor heater circuit testing:

How to check the oxygen sensor signal?

Difficulty: 3/5 Time: 30-60 minutes
Tools required:


  • Start vehicle and warm engine up to operating temperature
  • Using a scan tool or multimeter monitor the o2 sensor signal voltage, signal should be between 0-1 volt
  • Upstream (before catalytic converter) o2 sensor signal should be active and switch above and below 450mV as pictured below
Tip: Drivability issues and exhaust leaks can skew o2 sensors signals
  • Downstream (after catalytic converter) o2 sensor signal should not be as active and switch less than upstream o2 sensors due to the catalytic converter doing its job
  • Downstream sensors commonly stay around 500-700mv during steady engine speeds
Tip: A faulty catalytic converter will cause the rear o2 sensor to be active and cross count similar to the upstream sensor

How to test the oxygen sensor signal range

Difficulty: 3/5 Time: 30-60 minutes
Tools required:


  • Check that the oxygen sensor can measure rich and lean exhaust gas properly
  • A signal voltage under 450 mv is a lean exhaust gas and over 450mv is a rich exhaust gas
  • While monitoring the o2 sensor signal create a small vacuum leak and look for a quick drop in voltage (under 450mV)
  • Then perform a quick snap-throttle and check for a spike in voltage (over 450mV)

For more detailed o2 sensor signal testing:

  • If the oxygen sensor fails any of these tests, replace o2 sensor

How to replace oxygen sensor

Difficulty: 2/5 Time: 30-60 minutes
Tools required:


Oxygen Sensor Socket or 7/8 Wrench


  • Disconnect the oxygen sensor connector and release wiring from any retainers
  • Using a 7/8"" wrench or O2 sensor socket remove sensor
Tip: It may be required to heat the threads or use penetrating oil to remove rusty or seized sensors
  • Inspect threads for damage and repair if necessary
  • Install new sensor and torque to specification
Tip: Make sure sensor is fully installed before plugging the connector in to avoid twisting and potentially breaking the wire.
  • Re-install wiring and plug in connector

For more information, questions, or help diagnosing your vehicle, contact the GoTech technical support team via phone or chat below.

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