Hi - is there a common reason for the e/m light to just come on, was doing approx 4000rpm in third, and it came on

There is no misfire, temps and pressures are all ok, and everything sounds fine ............... fords want near on £100 to plug in and check , but was wondering if you can do like a scooby and check fault codes yourself for sensor etc???

If not ..... any pointers for anything to check that might clear this etc

Much appreciated Ry