Diagnostic methods and systems for electronic devices are disclosed. Separate dedicated diagnostic software code and operating code are respectively executed in a diagnostic mode and an operating mode of an electronic device. A determination may be made as to whether the electronic device is to be operated in the diagnostic mode or the operating mode when the electronic device is started, for example. A transition to the diagnostic mode from the operating mode may also be made in response to errors detected by online diagnostic tests performed in the operating mode. Results of diagnostic tests are preferably stored in a non-volatile memory for subsequent access, independent of the operating software code.