What is the best way to test repaired computers?

Always re-seat *everything*! It takes a little more time, but especially if the computer has been physically moved around recently, a stick of RAM could be jostled *just* loose enough that it *looks* okay but doesn't have a good electrical connection. Re-seating RAM, power cables, add-on cards, and data cables firmly ensures you've got a good connection. The only place I wouldn't do this initially is the CPU because of the time and risk in removing the cooler and then re-pasting/re-seating it; straightening bent LGA pins is a *pain*! If nothing comes on, test the power supply. If the PSU comes on but nothing on screen, *then* check the CPU. From there, use the BIOS to check voltages and connected peripherals.

I would like to add that It is crucial to conduct a functionality check after each individual alteration or repair. Without this check, it becomes challenging to identify the fault diagnosis when multiple repairs are attempted. Although it may require additional time, integrating proper troubleshooting practice into every repair is a valuable habit to cultivate. This approach will enable you to identify and rectify each fault, making you a more well-rounded technician and providing a better result to the client.

Make sure to check thermal paste condition and other items like CMOS battery as well. Also, depending on the model, confirm the CPU fan sits snug against the CPU, I've had a few cases where one of the secure clips is broken and doesn't sit flush.

Only thing I would add to what Jess said is that for older machines take the time when reseating the RAM and add-on cards to also give the slot a quick blast from your canned air. Also look at copper contacts on the RAM and Add-on cards to check for oxidation. If the contacts look dirty/faded then use a pencil eraser to clean them. This will help ensure good contact between the motherboard, RAM, and add-on cards.

Use the customer to show you the concerns, if they are not obvious. Verification and a visual inspection is paramount in the direction you go.

You can also examine the event logs to identify error messages, warnings, or other events that may indicate the cause of a problem, such as system crashes, application errors, or device failures.

As mentioned event logs are the easiest way to find descrepencies as well as programs like dxdiag and Windows/Microsoft updates.

Many of these should be based on the intended use to the extent practable. A graphics computer will have significantly different video needs from an audio editing or file sharing system.

The best tip is to maintain a standard testing protocol. Always listen for unusual startup sounds and inspect the BIOS settings. Check all external ports and ensure the casing/components are undamaged. Use diagnostic tools like Memtest86+ for RAM and HD Tune for hard drives. If you've repaired a specific component, stress test it thoroughly, one of the best tools is Prime95 for stress testing. Ensure connectivity, including Wi-Fi and Bluetooth, is functional. If applicable, monitor laptop battery health and system temperatures to detect overheating. Keep all drivers, firmware, and the OS updated. Consider a fresh OS installation if stability is questionable. Through process of elimination the root cause of the problem will be clear.

Thee are many software apps to test your repairs, apple uses a hardware tester. These tools should be used to test the repair but also stress the system to be sure you have not introduced some other issue or concern. I used to have the customer verify the repairs; they were the one who Had the issue first and they could usually repeat that issue. What a great tool they are; use them to attempt to repeat the issues. You will not already me the one doing all repairs; document and record any and all break/fix steps. Not only for you but for those coming after you. Place them in the cloud and share them for your team or create an FAQ.