Fridge is 4 and a 1/2 years old. A few weeks ago the freezer started to warm up followed by the fridge. I took the back panel off in the freezer and all the coils from top to bottom were caked in frost. I wielded the hair dryer and the fridge went back to normal for a few weeks and the same thing happened again.
I've done the hair dryer thing again and have a few days to fix it. I understand this is a problem with some element of the defrosting system: either the defrost heater, defrost limit switch or defrost timer.

How should I go about troubleshooting this? I'm moderately handy.
I don't have an ohmmeter or other testing devices. Is that a must?

Are there any other methods for me to determine the exact source of the problem?

Any pointers will be appreciated.

Thank you.

I would replace the timer first, just from what I hear.