Can you check an O2 sensor with a multimeter?

Yes, you can check an O2 sensor with a multimeter by measuring its voltage output. Start the engine and let it reach operating temperature, then connect the multimeter's positive lead to the O2 sensor's signal wire and the negative lead to a ground. A properly functioning O2 sensor should produce a voltage that fluctuates between approximately 0.1 to 0.9 volts as the engine runs. If the voltage remains constant or is outside this range, the sensor may be faulty.