Guide to Diagnose and Fix
When replacing a part, replace it with an exactly identical one. Some (all?) diagnosing jobs can only completed, if the part is exactly the same. 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, 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.
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. If the budget is very close (around 40$-50$ difference) to what that part's store price is, it is most likely 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.
PC Doesn't React to Powering Up, No Fans Turning, Nothing on Screen
- 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.
Fans Are Spinning and/or Lighting Up, But the Screen Is Blank
- 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.
PC Goes Past the Boot Screen, but Gives Error: No CPU/RAM/HDD Found
- 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.
The PC Boots Normally, with No Errors, but the Objective Stays Red as if Not Completed
- 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.
- 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: you swapped out a broken part with a different one
- Solution: Take out the part you "fixed", and install one that is identical to the original one. If you don't know what the original was, because you already sold it and don't remember... tough luck, reload an older save.