Jaguar xj 40 sovereign 1994 error code 17 - 1994 Jaguar XJ6

What happens if we don't change it?

I'm not a Jag expert, but error code 17 is:
P0017 Crankshaft Position - Camshaft Position Correlation (Bank 1 Sensor B)
Additional infomation on and cause and possible solutions are here:
http://www.obd-codes.com/p0016
Basically, it looks like you have timing issue. Could be the cam or crank sensor or the timing belt.
The P0017 code would imply that you have at least two cam sensors (V6 or V8). The computer thinks that the B sensor cam is out of time with the other cam and the crankshaft. Depending on the cost and location, you might try changing the cam sensor first. The cost to check or replace the timing belt is probably going to be painful.