Just bought a 1990 240 DL sedan and I'm not impressed with the gas mileage. on 1990 Volvo 240

Have read it may be the o2 sensor. Does anyone know how much that might cost to have looked at? or any other suggestions?

Asked by for the 1990 Volvo 240
possibly $100 to hook up a scope to your o2 sensor and verify operation...or just buy and replace it for $150 or so.