In addition to self-diagnostic tests that are regularly run by PCM some vehicles allow to run on-demand tests by scan tool request. This allows to see if certain fault code represents currently existing problem or is it rather a record of the fault condition that was present earlier but not existing at the time of test. Also in order for PCM to detect certain fault conditions engine have to be operating in specific conditions that might not be encountered often. On-demand running test allows to reproduce these conditions whenever it is needed to detect problem.
KOEO stands for “Key On Engine Off” and performed with ignition turned on but engine not running. Generally this test is sufficient for detecting problems with electrical circuits.
KOER stands for “Key On Engine Running” and performed with engine running at normal operating temperature. This test can detect most of the fault conditions as long as they are present on the vehicle at the moment of test.