Car model and year: Porsche 996 1999-2005
Purpose: to read the cel error(s)
What to do next?
A standard OBD2 reader will be able to read the CEL code.
One has to wonder, if it was already diagnosed, and it's just an O2 sensor, which is easy to replace
An OBDII read works for code reading on a 996. O2 sensors are easy but a code related to O2 sensors could point to a whole series of other things related to air/fuel or other maintenance depending on the exact code.
First of all, it's good to know you're using good due diligence in this case and didn't fall for the banana in the tail pipe. Many people will claim the O2 sensor needs to be replaced when actually the sensor is doing it's job to alert about an emissions related fault. I've been down this road myself. I tried to convenience myself it wasn't the cats, but after throwing money at non-related things (i.e. new gas cap, new oil filler cap, new O2 sensors, etc), in the end, I had a bank#2 cat going bad. Replaced it and the CEL went away. Yes, it was expensive to replace, but it more expensive than it needed to be because of my denial.
Maybe you are interested in Foxwell NT530 / NT510 Elite Porsche 996 function list: