OBD II trouble code

I went for inspection and they told me my emissions didn't pass and gave me a paper stating I got a OBDII PO134 trouble code. I know it has to do with emissions. So where should I start?

My catalytic converter is old but i'd rather mess with the smaller stuff first. My Neon book says to use the ohmeter and read the voltage to all related sensors. thats what I had in mind any help would be great thanks again AF.