Technician A says that a lambda greater than 1 means the engine is running rich. Technician B says that the amount of oxygen in the exhaust indicates how rich or lean the mixture is. Who is correct?

Technician A is wrong , Technician B is correct.

Explanation:

The Air fuel ratio is used in the combustion process of engines whether internal or external, and defined as the mass  ratio of air to  fuel present in a combustion process.  When the ratio of the mass of air and fuel is right, combustion is said to be efficient and balanced and this is refereed to as stoichiometrc mixture which is a standard that all engines are required to target.

when the air fuel Ratios is  lower than the target stoichiometric, it is considered as a Rich mixture  while higher ratios  than stoichiometric are considered Lean mixtures. Lean mixtures are more efficient than rich mixtures although the main aim is to be at the stoichiometic mixture point.

Air fuel equivalence ratio, lambda and the amount of oxygen in an exhaust are two indicators for lean or rich mixtures in an air fuel ratio for combustion in engines

Now, Air–fuel equivalence ratio, λ (lambda), is defined as the  ratio of actual AFR to stoichiometry for a particular mixture,

when Lambda λ = 1.0 , the mixture is at stoichiometry,

when  λ < 1.0, the mixtrure is rich.

when  λ > 1.0, the mixture is  lean

 Therefore Technician A is wrong

Also The amount of oxygen present in the exhaust determines if the air-fuel mixture supplied to the combustion chambers is at the target or best ratio. Too much oxygen indicates that the mixture is lean and too little  indicates the mixture is rich. Therefore Technician B is right.