Code 53 - Diagnostic Monitor Internal Fault: Description
WARNING: This page is about a different car, the 1995 Ford Pickup, 1995 Ford F-Super Duty, 1995 Ford Cab & Chassis, and 1995 Ford Bronco. However, it is still accessible from the selected car via links, so may be relevant.
Diagnostic monitor performs several self-tests every time ignition is turned on. If any of these tests fail, Code 53 will set.