is it ok to use a universal o2 sensor?

[racerchaser]

I've been shopping around and pricing the pre-cat 02 sencor for my 03 wrx. The dealership will charg $190 which is the best price I have found. But I have also found on some online parts stores the same sensor, but it is a universal sensor so I'm sure it doesn't work quite as well. They are only charging $33 for theirs. Does anyone know why they are so cheap? What is the worst that could happen if I put on the Universal? Any input would be great. Thanks!

[TrackReady]

Do yourself a favor and don't use a universal one. Especially for your up-stream o2. All O2 sensors switch between around .1volts (lean) and .9volts (rich). This is how they can make a "universal" o2. Problem is these universals will almost always "read" slow. The car is continously trying to make the sensor "switch" from rich to lean. Universals will switch, but they will do it stubbornly and the car will not run up to peak performance, and might cause drivability problems. Why this happens exactly I couldn't tell you. It probably has something to do with possibly a sub-par metal causing a slower chemical reaction. O2 sensors change there resistance through a chemical reaction that differs by the amount of oxygen on the external part of the sensor compared to the oxygen present in the stream of exhaust

[kgelster]

Pre-cat O2 sensors determine fuel trim. Use the OEM sensor since that's what it's calibrated for.
Aftermarket is fine for post-cat sensors.

[racerchaser]

I appreciate your input. I went ahead and cracked open the ol' pig for some etra cash and bought the sensor from the dealership. I hope I diagnosed the problem correctly, it was a very expensive purchase to waste.