Fitted a new chip/cloosed loop to my car .. Escort Cosworth.

Because my last chip was a very generous fuelling chip I havent been concentrating so much on the economy and if the lambda worked ok.

My question is about if my lambda sensor is dead or not...

I will not see anything like 14-14.3.4.5.6.7 afr when cruising with lambda connected (tried 3 used lambda sensors) when disconnecting my lambda my afr figure drops from about 13.5 to about 12.2-12.5. The same sort of drop in afr happens at idle and normal/no load driving.

Can a lambda sensor be out of order, but still working but "just" being out of range, not regulating enough??

How can I be sure that the lambda is working CORRECT.. or not?

Another thing is.. can a lambdasensor tester check a lambdasensor 100%, and how does such a tester work??