Just had the car at the garage they did a diagnostic and it came back with 2 codes, defective front lambda sensor and circuit knock sensor.... Anyone think they can explain this, should I replace or go let someone else have a tinker? the juddering and misfiring is still bad at times and another guy told me it could be blocked injector??? Getting right on me nerves now, 40 quid to be told what it 'could' be!