Hi, I bought my KA from a garage on the 30th April 2010, It is an 04 plate and had done 55k miles. Since having it I have had to take it back to the garage I bought it from 3 times as the engine malfunction light kept coming on. As this has now happened a 4th time I decided to pay to have it looked at in another garage. This garage came back with the fault codes:

P1631 - Smart alternator faults sensor circuit malfunction.
B1600 - Passive anti-theft system ignition key signal not recieved.
P0620 - Generator control unit malfunction.
P0420 - Catalyst system efficiency below threshold.

The garage could not fix these problems and have told me to go to the Ford garage, which I plan to do, but he also said it would cost a lot of money so I am saving up. The garage have turned the fault light off so I am also waiting for it to come back on.
If anybody could explain what these codes mean and shed some light on how much money I could be expecting to pay, and if the car is worth fixing.

Thanks a lot

Polly