I've had the same problem with my 406, you must not be scare mongered into decisions when you are not sure
The most insignificant fault will cause the light to stay on.
Fault finding is an art and you should always double check with two sources if they suggest something expensive.
In the end you must find a smaller diagnostics garage that will not charge you a load of fixed charges
Have the experience to interpret the information is a skill that is not that common
David Mishra
February 2010