Problems that are usually repairable
A deadbolt that drags because the strike is misaligned, a handle that has loosened, or a cylinder that needs cleaning and lubrication can usually be fixed in one visit.
Winter-related issues
Exterior locks in the GTA deal with moisture, ice, and salt. Use a dry graphite or lock-specific lubricant rather than oil, and have the door alignment checked when seasons change.
Signs it is time to replace
Cracked housings, corrosion inside the cylinder, worn internal parts that are no longer available, or a lock that has been forced are reasons to replace rather than repair.
Upgrade while you replace
If you are replacing anyway, consider a higher-grade deadbolt, a reinforced strike plate, or a keypad lock. Have all exterior locks keyed alike for convenience.
Get it inspected first
A locksmith can tell you on site whether a repair will last. Ask for both the repair and replacement price so you can make the right call.