58 plate 2.0 Mondeo . Engine warning light came on took to garage they put it on a diagnostic and thought was cured .

The warning light more times than not is coming on because of the emission system. There are several sensors that are hard to diagnose even for a professional with a computer. First place to start which is cheap and easy is to use a fuel treatment for a few tanks of gas. This has worked for me in the past by cleaning out the fuel injectors and fuel system. From there if this does not work you will have to check O2 sensors everywhere they are and replace.