Hi Pete, on the 3 cylinder 1.0 corsa's when you clear the fault codes you also reset adaptions, i suspect yours does the same as when you cleared fault codes the engine management went back to basic settings & thats why O2 sensor suddenly read lean, if you looked at stft & ltft they would also of change to suggest the ECU was having to add fuel. I also think this car is running lean & also suspect airflow meter as i have had changed quite a few for running lean. Hope this helps Darren