The Check Engine light comes on usually after a highway drive of 100 km or more. It then goes off. The fault codes " Multiplicative Mixture Adaption - Bank1 and Bank 2" are recorded. I have changed the O2 sensor, plugs but it still repeats. There is no noticeable reduction in fuel economy and no symptoms when running.

Any suggestions?

Check for vacuum leaks around intake. It must be something that affects complete engine because you have faults for both banks

you might want to check the air flow meter