I have been told that most cars, 2007 and up have now a computer system that go through self diagnosis, and this is done every time a vehicle is driven twice for a certain distance. It is said that this is what is called the two trip logic. I need an ASE certified technician to verify this and explain how this works. Please state your credentials to verify your experience. Thank you.
The two trip logic is that the PCM will scan the system for one trip( key cycle). If a trouble or problem is found on the first trip it won't turn on the check engine light but will keep the code in the PCM memory on this trip (key cycle).
If the problem occurs on the second trip (key cycle) then it will turn on the check engine light. This procedure in the PCM programming helps eliminate false codes that may only happen every once in a while.
There are some more important codes that will turn on the check engine light on the first trip (Key cycle). This is so the vehicle can be diagnosed before any damage can be done to the engine, transmission or vehicle.
I am a certified ASE mechanic for 35 years and am now retired.
Self-diagnostics have been around long before 2007. It began with OBD (On-Board Diagnostics) in the early '80s. And these self-diagnostic tests are not simply performed with every trip. Certain criteria must to met before enabling each test. For example: to perform an evaporative emissions test the PCM must see engine coolant temperature, intake air temperature, and battery voltage at a certain point before the test will even run.
Consumer concerns on self-diagnostics really began in 1998 when states began utilizing the OBDII system as part of motor vehicle inspections. The systems are scanned for "Readiness Codes", meaning the status of self-diagnostic tests which have a direct affect on emissions output. They are either incomplete, complete, or failed. If a certain number of tests come up failed or incomplete, the vehicle fails inspection.
And a common issue is a dead or disconnected battery. This clears the PCM's memory, requiring it to re-run all the diagnostics tests. This can sometimes take many miles, key cycles, driving conditions, and temperature changes to accomplish.
And like TechHelp2 was explaining, certain failures which will set a DTC require the fault to be seen by the PCM for 2 trips before turning on the MIL, and some will set the warning light immediately with the first sign of a problem. But this is not always a part of self-diagnostics of a sub-system, but just the PCM monitoring individual sensors, components, etc.