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[GrampyJim]

Right... retrieving memory codes is just what it says... we are retrieving codes that are stored in memory. The concern MAY or MAY NOT be present at that time.

There are two basic self tests that can be (actually "should" be) run. The KOEO On Demand self test instructs the PCM to perform a preprogrammed self test. This actually checks sensor data and operates some actuators to see if an identifiable problem is present at the time the test is being run... These "hard" or "on demand codes" must be addressed before any memory codes.

Now, some sensors and actuators cannot be accurately tested without having the engine running... hence, the KOER on demand test... During this test, a similar routine to the KOEO test is run. During this test we can hear things change... the engine will speed up, slow down, get quiet, get noisy.. all this as the PCM instructs a change of some sort and watches for a correct reaction to the change.

Some of the codes retrieved as "memory" codes can become on demand or hard codes during this test (just to cloud the issue, some memory codes are "instructional" codes... simply indicating that something didn't look right but damned if the PCM knows what it was).

Again, on demand codes should be addressed before any memory codes.

The heart of the matter is that the scan tool will offer the option of running these self tests... without this option and without telling the scan tool "yes, I want to run this test"... the test will not be run and you will be stuck with memory codes....

"Why are memory codes so bad?"... Well, they aren't.... but they aren't great, either. It's like having the smell of a great pot of chili coming from the kitchen and getting a bowl of beans at the table. If we leave a concern for long enough, conditions can degrade to the point where the PCM loses control of other things.... these can set codes of their own. Fix the original concern, and you might never see these other codes again... Unplug a sensor with the key on "just to see what happens"... and there's another code or perhaps multiple codes.... Thus, the recommendation to clear memory codes (in the case of multiple codes) and see what comes back first.

Just like running the KOEO and KOER tests (we have to "tell" the PCM we want to do this), running the On Demand tests will not erase any codes... a confusing part.... we can run an on demand test once and have a <particular> on demand code.... We can rerun the same test moments later and, if the concern is intermittent, that code can disappear into thin air... making it look like the code has been cleared.

Other valuable information from the On Demand tests includes "freeze frame" data (the operating conditions present at the time the code was set) and "pending codes" (conditions that have made the PCM sit up and take notice but haven't yet fulfilled all of the requirements to be announced).

If it was all as simple as some appear to think, guys like Doc, Greg and all the other dealer techs that give freely of their time and knowledge would be sitting in the SallyAnn soup kitchen for lunch.