1992 Chevy Astro Van TBI 4.3L LT sub model. Why is the o2 sensor getting hot?

had same problem in my 92 astro. my err. code was fuel pump relay. cranked normal, no fuel. mechanic had to run a separate wire from PCM to fuel pump relay. as a short occured in the existing wire. Also had to replace blown PCM (computer), and O2 senssor.