OBD1 codes 25 and 26 show up causing the check engine light to turn on?

Van runs fine, starts right up. It does have an exhaust leak maybe at the manifold. Can hear it but it is not sounding like a hole in the exhaust system. OBD 1 Codes 25 and 26 say too rich mixture and too lean mixture. Seems to me it should be one or the other. Will take it to a smog shop but if you have a recommendation of what I can do before hand I might be able to save on mechanics repair time. I don't smell unburned gas smell when it starts and runs. Might be not getting good gas milage. I guess the manifold leak is causing both codes due to the leak the sensor is feeding more fuel and the o2 sensor is sensing too rich mixture and the cat will be damaged if the exhaust manifold leak is not fixed.