Engine management?

The engine light is permanently on in my Ford Ka and using an EOBD reader I have the fault code P0171(Mixture too lean) I have checked all hoses for leaks,it has a clean air filter and new plugs. I am trying to avoid a costly trip to the garage so they can charge £50 just for using their diagnostic equipment.

I assume the next step is somehow testing the two oxygen sensors either side of the cat.

Any help would be appreciated