Primary O2 is on the exhaust manifold on my '99. Should be in the same area on yours. Follow wires to plug and then to harness. Make sure wires are plugged in/not broken. I think I remember that the old O2 sensor plugs (had '91 CRX) were kinda flimsy. The shop may have pushed a contact out of the plug. O2 sensor is not as fragile as the automotive industry would have you believe. Your car is also running full rich right now. This will cause you more O2 sensor problems.