What does a check engine code PO136 mean for a 1997 Toyota Coroll?

My '97 Corolla has the same problem. It's an oxygen sensor that's malfunctioning, according to the little plug-in code reader I borrowed from AutoZone. It says "bank 1 sensor 2." That means that the bad sensor is downstream from the catalytic converter, so if you want to replace it yourself, you'd have to somehow get under the car and switch the old one out with the new (this would be a lot cheaper than letting a shop do it for you). Hope you get things worked out.