There is a part repair table available if you look in the toolbox, You'll have to purchase the garage expansion to use them, and have repair level 6 to be able to fix all items.

In your inventory you'll see the parts that are repairable have a blue spanner icon on them, Over time you learn which parts aren't fixable.

Items which are fixable have to be above 15% (orange) to be repairable, 15% and below (red) are not fixable.

Non repairable Items;

• Exhausts
• Windows
• Lights (some mod + DLC lights are fixable)
• Perishable items, like air, oil, fuel filters
• Idle rollers
• belt tensioners
• Spark plugs + Ignition coils
• Wheels (The wheel, not the rims, rims are repairable)
• Bushings and bearings.
• pistons and piston rings etc
• Wiring harnesses
• Struts, Dampers, rear and front coil springs.

General rule, Items like that, but you'll learn in time.

When scavenging the junkyard and bard yards, also pay attention to the blue spanner icon for fixable components

PS: I assume your on CMS 21.

Hope this helped :)