Like all sophisticated systems, Macs do sometimes go wrong. Indeed in some hands they always seem to be teetering on the brink of going wrong.

In the many years of writing Q&A columns for MacUser, I have used a common set of techniques to diagnose problems from which readers’ Macs were suffering, and arrive at solutions. Here is a distillation of generic troubleshooting techniques suitable for OS X 10.10 Yosemite, in the hope that it will one day help you.

Startup problems

Prior to the delivery of OS X from the App Store, and OS X’s use of hidden recovery partitions, the standard methods of working around startup failure included using an external (or internal) bootable drive, and starting up in Single-User Mode (SUM). Since OS X Lion, the mainstay is to use the Recovery System, which is best augmented with a prepared USB stick containing your preferred recovery tool, such as Drive Genius 4 or Disk Warrior.

Essential startup key combinations include:

• Apple Hardware Test (AHT) — D key
• Select the startup volume — Option key
• Single-User Mode (SUM) — Command and S keys
• OS X Recovery System — Command and R keys
• Safe Mode — Shift key
• Eject removable media (CD/DVD) — Eject or F12 key, or hold mouse or trackpad button
• Reset NVRAM (PRAM) — Option, Command, P and R keys until second chime.

Apple’s full listing is here. It is most useful if you keep a wired USB keyboard and mouse handy in case OS X struggles to load the drivers necessary to use wireless peripherals.

Apple’s Recovery System can restore your Mac from a Time Machine backup, let you run Disk Utility to check and repair the startup volume (or any others currently connected), install or re-install OS X, and will walk you through testing an Internet connection if you need. It is documented fully here.

There is an important difference between re-installing OS X using the Recovery System on the hidden partition of your Mac, and using Internet Recovery. The former re-installs the most recent version of OS X found on your Mac, but Internet Recovery takes your Mac back to the version which originally shipped on your Mac, leaving you to update using the App Store.

Crashes, freezes, coughs, and wheezes

You may be able to make these go away by restarting in Safe Mode (Shift key), which disables third-party add-ins and flushes many of the system caches. If so, suspect an old third-party add-in which needs to be removed or updated.

Another good approach is to create a new user (with admin rights as necessary), and log onto that user account. If the problems vanish, then they probably relate to something nasty lurking in the ~/Library folder of your normal user account, such as a third-party add-in.

Think back to anything that has been recently installed or update, as that may give a clue as to what is the cause of these problems.

Removing third-party products can be very difficult at times, as some such as ‘cleaning’ and ‘housekeeping’ utilities seem tenacious and designed not to let go. Look for an uninstaller, or try running the product’s installer with the Option key held down, which can sometimes engage an uninstaller. Search online for documentation or hints as to what needs to be removed.

Console is a key tool for getting better clues as to what is going on. Open it immediately after a problem has occurred, and with All Messages selected in the left pane, browse back to the time of the problem and inspect the log output. Crash logs are much less useful in user diagnosis and intended mainly for developers, but the main log entries often contain valuable information. You can sometimes select a portion of the log entry and search on the Internet to discover others with the same problem, which may have been solved in a discussion forum. Occasionally such searches even take you to a support note explaining the problem.

Poor performance may be reflected in the logs by a vast list of identical crashes of a key background service, which is then automatically restarted. These are distinctive entries which usually refer to the broken service being ‘throttled’ or killed, and launchd (or similar) starting them up again, or ‘respawning’.

Logs are also valuable for checking problems in Time Machine, whose backups are marked periodically in the logs, and things going wrong during the startup process, if you inspect the logs straight after a restart.

Activity Monitor can be helpful, but is actually harder to interpret in most cases than the logs. This is because OS X manages resources to make the best use of them. If you see most of your physical memory actually being used, that is a good sign, as it improves performance, and makes effective use of the hardware available. Sometimes ordering the list of processes by %CPU can reveal a process which is chewing up processor time, but be very wary of drawing any conclusions from the Memory view.

The Network view can show processes which are making heavy network demands, and can be a substitute for the likes of Little Snitch for detecting which apps ‘phone home’.

You can force processes to quit in Activity Monitor, but if you have a frozen app, press Command, Option and Escape to bring up the Force Quit Applications dialog. When processes or apps are forced to quit, OS X should clean up properly after them, but it is often wise to restart as soon as practicable. This allows OS X to check hard disks and make any minor repairs necessary, and ensures that all detritus is cleared.

Disk and permission woes

OS X’s built-in disk checking and repair systems have reduced the problems which used to result from minor disk errors, readily leading to cumulative damage and an ultimate major crash. However when you are experiencing problems it is often wise to ensure that your startup volume is fully healthy using Disk Utility.

If you are convinced that a third-party disk checking and repair tool must be better, there is now a wide choice. DiskWarrior is particularly good for addressing directory problems, and now copes with modern, large volumes. On occasion it can recover disks that Disk Utility gives up on. Drive Genius 4 is a proven all-round disk repair and diagnostic tool.

Most internal drives should have support for S.M.A.R.T. diagnostic checking, which can give advance notice of a major failure. Status is normally shown in Disk Utility, but you can use third-party tools to give greater detail when needed.

The old panacea of repairing permissions in Disk Utility does still sometimes help, and is worth doing after you have checked drive health, if you are at a loss for a better idea.

Strange problems can also result from odd file permission settings. Select the folder or file in the Finder, and use the File / Get Info command to show permissions, changing them as necessary.

Network and Internet

Problems with Internet connections are often diagnosed using the status lights on your modem-router and its browser interface. Refer to the device’s documentation for further details. You should also be checking the logs there at least once a day, and they are a good starting point if the connection keeps dropping.

If your Internet connection appears OK, but things seem to be going wrong beyond that, use Network Utility (in /System/Library/CoreServices/Applications) to ping a remote site by its regular domain name, such as http://www.apple.com, or its IP address. Network Utility contains many other valuable tools which can diagnose network problems, and should be used in conjunction with a check on IP addresses in all the devices connected to your local network; for Macs, use the Network pane in System Preferences.

Wireless networking can be very hard to diagnose, but a useful first step is with Wireless Diagnostics in /System/Library/CoreServices/Applications. The more generic tool Network Diagnostics, in /System/Library/CoreServices, can also help, although it may not be as useful as more specific tools.

Getting lower down with more complex network problems quickly gets very messy. Experts resort to packet sniffing using a tool such as WireShark.

Fonts

Unless you have a third-party font management system installed, the key tool for checking and managing fonts is Font Book, found in Applications. Select them all and use the File / Validate Fonts command to check for duplicates and damaged items.

Preference setting not sticking

When an app or pane’s preferences seem to be unchangeable, in that you try to change them to do what you want but they keep reverting to another setting, locate the property list containing those settings, usually in the ~/Library/Preferences folder, and move that .plist file elsewhere. You will then have to set up its preferences afresh, but they should stick properly now.

Apps failing to start

If you try to start a Java app but it refuses, you may not have a suitably updated version of Java installed. Now that Java is not part of the OS X bundle, you will need to visit Oracle to download the current installer.

All apps should now be signed by their developer. Depending on your settings in the Security & Privacy pane, General tab, this may prevent some signed and any unsigned apps from opening. If you have downloaded an app which cannot be started because of this security mechanism, first ensure you have that tab set to allow apps from the App Store and identified developers. Then, instead of double-clicking on the app to open it, do so using the Finder contextual menu and select the Open command. This will give you the option to run it. This only applies the first time that you try to run the unsigned app: thereafter OS X will recognise it as being OK. Only do this if you are certain that this is a genuine app, and not a Trojan or other malware.

If you double-click on a document and the wrong app tries to open it, try changing the extension to the document name, for instance to .txt or .text if it is a text document, or use the Finder’s Get Info command or contextual menu to select the correct app. You can also use the latter methods to change the app for all documents with that file type (extension).

Forgotten passwords

If you have used a login and password before, it may well be stored in your keychain. You can now check those entered via Safari via the Passwords tool in its Preferences. If you cannot find it there, or may have used it without Safari, open Keychain Access in the main /Applications/Utilities folder to look for it.

Printers, scanners, and other peripherals

The golden rule here is to check that you have the latest support software, including drivers, installed. If in trouble, uninstall the printer or peripheral, and re-install it. Some apps may fail to print, but others still work OK: one workaround is to print to a PDF file, and then print that using Adobe Acrobat or Preview.

Other tools

Some of the utilities which used to be exposed in the Applications or Utilities folders are now hidden away in /System/Library/CoreServices. If you cannot find a tool which used to be available, look there next.

Very occasionally it is simpler to correct a problem in the command shell, Terminal. Do not be afraid: check everything that you type in there at least twice before you press the Return key. Some commands will only work when run as the root user. You normally achieve that by prefacing the command with sudo. You will then have to type in your admin user password at the prompt, before the command will be executed.

In many cases when Macs seem to be getting a little odd, simply restarting does the trick, by clearing whatever whatever was responsible. Keep a note of suspicious circumstances as it may help you understand a more persistent general problem.

Finally, please report bugs and problems to the responsible software developers, so that they can be aware and fix them.

I wish you success…