For various reasons I disconnected my battery and when I next drove the car it would hardly run, the engine misfired and was gutless. I was about to take it to a dealer when I read an obscur reset procedure for the engine management unit. Hey presto it worked and the car is running fine now. This might only work for v6 models but you switch the ignition on for 30 seconds (don't start the engine) and then remove the key for a further 5 seconds. Put the key back in and start the engine.