Just an interesting little tidbit: there is a system test built into the EFI on MacBook Airs and MBPs - most Alu MBs with an Intel processor. If you hold F2 and start the computer, it will drop into hardware test version 3A152, which is apparently right in memory. This saved me when I didn’t have my MacBook Air start-up disk available.
If the F2 key does not work for you, hold down the D key. Apple appears to have switched to using the D key for both Install Disc HTM and On-drive HTM.