1996 GMC Safari code P1133

Had this GMC(4.3 L) van been sitting for a while, recently started it up for a smog test, but check engine light stays on, pull the code says p1133 manuf air fuel ratio mixture. So I replaced the pre-cat O2 sensor, reset the code, but check engine light came on again after about 20 miles of driving, should I replace the other O2 sensor togeter with the first one? I wonder what the real problem to this check engine thing. Thanks for any professional mechanic advises.