What does "HEGO" stand for?

I did a self test on my '89 gt vert today, and I got a "91" code, which translates to "No HEGO switching detected" according to Chilton's. Anybody have this problem? Where is it and how do I fix it? I'm guessing it's
H***-Exhaust-Gas-O*** but if anyone could tell me I'd appreciate it.

BTW: I also got a code "31" related to the EGR valve system ("PFE, EVP, EVR circuit below min. voltage"), and either got another "91" or a "19" during the engine running test(related to HEGO or EGR respectively-Im gonna try it again tomorrow) if that helps you guys any. thanks again

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[This message has been edited by Big&bad89rag (edited 04-05-2001).]