I have a 2005 Hyundai Accent, and the check engine light is on. I'm almost 100% sure it's the O2 sensor because it's not as fuel efficient as usual... My work schedule conflicts with mechanics around me, and I do not have time to get a reading on the car. Is there a way I can identify whether it's the upstream or downstream sensor that needs replacement?
This requires a bit of mechanical know-how and some time...
An O2 sensor can be tested with a multimeter that measures millivolts. You'll need to securely attach one lead to the signal wire of the sensor and the other to a good ground on the car. This all has to stay in place with the car running...
Fire up the engine and look at the voltage reading of the sensor. Low (lean) should be around 0.2v and the high side (rich) should be around 0.8v. Rev the engine repeatedly and see if it fluctuates. You can also force a lean condition by allowing air past the MAF by unplugging a vacuum hose or just loosening the MAF from its boot or force a rich condition by obstructing the intake.
Test both sensors and see if one fails to respond. If they both stay at the same voltage (report always lean or always rich) then there could be another problem, but if one responds and the other seems out of wack, it's probably the problem.
Warning: The exhaust and engine get very hot, very quickly. Attaching the multimeter to the O2 sensor connector could also be tricky. If the O2 sensor has 3 wires it's a "heated" O2 sensor, and you need to find which of the three is the signal wire.
You may want to review this answer of mine.
The tests outlined there for heater resistor continuity and response to lean/rich conditions assume a fully functional sensor. If the behavior under testing doesn't match what is outlined there, you know that the sensor has an issue.