This workflow has originally been made and posted on the Discord channel by Mathijs. I've worked on it a bit and I hope it will be helpful to you guys. Click on it to get a link to the full size version, click on the link to actually see it in full size.

When replacing a part, replace it with an exactly identical one. Jobs can be completed with "better" parts, however the standard for what counts as "better" varies between component types, and instead of memorizing the standards or looking for each property of the item being better or the same, using the same component is easier. I also recommend buying a new part for it, to absolutely rule out the possibility of losing a star due in case the customer wants new parts. If you have a brand new one in the inventory, you can use that, of course. Using a used part doesn't save you money, since you get reimbursed for its value either way, in which case you could make way more money with that part on PCBay. Keep in mind, that the budget will ALWAYS cover buying a brand new one of the broken components, with 25-40% to spare (my approximation, not fact).

Reject jobs that want you to diagnose and fix their PC, but has the ASAP hidden objective. If you don't know all the defective parts when you accept the job, you might be forced to buy the replacements with same-day shipment, if you don't want to lose the star for speed.

If you're completely stuck with your diagnosis, take the computer apart completely. Every working part should have a yellow stick-it on it, marking it as part of this computer, and what's left, either broken or mismatched (not the correct replacement part), are the parts you need to pay attention to. This, however is a last ditch method, since it takes way more time than correctly diagnosing and fixing the problem.

If the email mentions a part manufacturer as preference, look at the PC Stats in the email, to see which part is from that manufacturer. Most (all?) of the time that part will be broken. When the customer mentions the name of a part (like "Did my PSU break?"), that part is surely broken. If the part price is about 60-80% of the budget, it is likely to be the only broken part, and you can order for next day before the PC even arrives.

Frequent saving is a good idea. I usually save before turning in a job, to have a save just before I screw up a star. I delete obsolete saves created like this whenever I completely run out of jobs - in this way I always have a revert point for before buying parts too (in the worst case, the last save before accepting a new batch of jobs). You can also save-scum by accepting a diagnosis job, saving the game, going to next day, stripping the computer completely, taking a note of the broken parts, then reloading the save and buying the parts in advance. Note, however, that even though this way you can do jobs more quickly related to in-game days, in real time this results in slower progression (less money and xp), unless you're really bad at identifying broken parts. The one advantage of this method is that you can take note of other parts that can be swapped out too, and possibly make a bit of profit on them, however keep the budget in mind. I generally don't consider this being worth the effort.

Possible problem: broken PSU
Solution: Take out the PSU. Look at it in the inventory, if it is broken, order a replacement.

Possible problem: broken motherboard
Solution: Take out the motherboard (check the PSU first since it's easier to take out). Look at it in the inventory, if it is broken, order a replacement.

If you have previously worked on the PC, check that the case, the fans and the GPU is connected to the motherboard, otherwise it can produce the same symptom without these parts actually being broken.

Problem: broken GPU
Solution: Take out the GPU, check in the inventory that it is broken, order replacement.

Make sure you have the GPU plugged in to the PSU and the monitor, otherwise it can produce the same symptom while not actually being broken.

Problem: The component mentioned in the error is broken
Solution: Take out the mentioned component, check in the inventory that is broken and order a replacement.

Bear in mind, that there could be multiple RAM sticks or hard drives in the PC, and you don't know which one is broken until you check in the inventory. In case of hard drives, if there is a HDD and an SSD, it will usually be the HDD. Also make sure that the HDD was plugged in to both the motherboard and PSU, since not having it plugged in can also produce the No HDD found error.

If you swap out the HDD, you might get the error "No OS found", in which case you need to a USB stick plugged in while the PC is booting (restart if you already have the error screen) and the OS (operating system) will be installed automatically.

Possible problem: there is still something to fix
Solution: check for parts that can have several more installed: HDD, SSD, RAM, GPU, fans. Most of the time people miss an additional HDD being broken. Take out the broken part and replace it with a new one.

Possible problem: virus
Solution: Plug in a USB stick, start the PC, then install the Virus Scanner app. Restart the PC, then run the Virus Scanner app (double click), scan for viruses, and if it found a number of infected files, press the "Clean Files" button. If there is no such button after the scan, and the number of infected files is 0, the PC didn't have a virus.
Also, if you've taken out the drives to check if they're broken, but neither of them were, but you had to install the OS afterwards, it means you changed their boot order, switch it back from the last tab in bios and run the virus scan on the original drive too.

Possible problem: dust
Solution: Take the Compressed Air from your inventory, point it at some part of your PC and blow it for half a minute. Don't move it around, trying to aim at specific parts, just blow it in one spot.

Possible problem: no thermal paste or CPU cooler broken
Solution: When you power on the PC, check if the CPU cooler is spinning/lighting up. Take off the CPU cooler. If it wasn't spinning/lit up, check to see in the inventory that it is broken, and order a replacement. If it is not broken, or you already have the replacement, in the first (Tools and Software) category, select the Thermal Paste. Click on the CPU to apply it, you should see a silvery white smudge appear on it. Put the CPU fan back (don't forget to connect its cable).
When checking if the CPU cooler is spinning, make sure that it is connected to the motherboard. AIO coolers only have to be connected from the block on the CPU, not the radiator.

Possible problem: radiator or fan broken
Solution: Similarly to CPU cooler, check if all the fans in the case are spinning. If some are not, and you don't see the Missing Cables error, that fan is broken, buy an identical replacement and replace it.

Possible problem: you took out something and you didn't put it back
Solution: This happens when there are multiple drives (HDD, SSD, M.2), GPUs, RAMs or fans in the PC. Check your inventory to see if there is something you forgot to put back.

Possible problem: you swapped out a part to a different type
Solution: This happens when you install an AIO cpu cooler instead of the original air one, or an SSD instead of an HDD (or backwards, but that would usually give an inferior part warning instead). Take out the part that you swapped, buy one that is the same type as the original (and not inferior to it, optimally identical), and install that instead.