1999 Jaguar XK8
Will code P1646 cause the car to go into limp mode. Clearing the code will only fix it for a very short time, like about 4 miles. Code P1000 also appears.

 

The code would indicate an A Bank upper oxygen sensor failure and would set limp mode. The P1000 just means that all monitors have not passed and will change to P1111 once everything is passed.
With the sensor failure it inhibits bank A to protect the catalytic convertor.

One of the monitors will probably drop by and transfer your post to the X100 area which is the appropriate place for questions for your XK8