1997 GMC Sonoma service engine soon

Running rich means either the vehicle is dumping fuel, or not burning the fuel that's in the cylinder, or the catalytic converter isn't burning off the excess. Id start with checking/changing the plugs and wires. Then check for a fuel injector sticking open, then check the "in" temp against the "out" temps on the bank two catalytic converter. It should be much hotter on the "out" side