Very handy to have a reader that works on any vehicle made after 1995.
Help for friends and family as well, since pretty much everyone has a car/truck, lol.
Code numbers are written in computerese, they assume the person looking at the code number understands how simple computers "think", and vehicle computers are very very simple, they are basically a low speed calculator, 15-45MHz speed.
Your Iphone runs at 1.3GHz speed, that's 1,300MHz
So in the computer world this is a very very slow and simple calculator, vending machines are "smarter", lol.
But these types of computers last a long long time, and do a good job.
Because of this limited "mentality" the computer sees everything as black or white.
It has parameters(ranges) in memory, if it checks a parameter and it is out of range then it will store that parameters code in memory, when it checks that same parameter again and it is out of range again it will store code again, this will go on for a while until computer reaches another parameter that tells it to turn on the CEL to notify driver of the repeating code number, or numbers, and it is different for each code number, so it isn't "if 3 times" turn on CEL, could be 1 time turns on CEL or 10 times turns on CEL, that part is set by the program in the computer.
So it isn't an immediate "out of range turn on the CEL" reaction.
All the sensors or monitored devices in the vehicle will go out of range once in awhile, that's just the nature of electrical devices in a variable voltage system.
So computer looks for repeating out of range issues before reporting it to the driver.
The exceptions to this "rule" is emissions systems, these codes will come on first time and every time, and are very hard to clear.
The clearing part is "drive cycles", and this applies to all codes.
When engine is cold computer runs in Open Loop, the is 1/2 a drive cycle, Open Loop means computer can't use the O2 sensors to control air/fuel mix, O2 sensors don't work until they are heated above 600degF, so computer runs engine using pre-set table of air/fuel mix based on RPM, TPS and MAF data, computer is also running a Choke Routine, rich fuel mix, high idle and advanced spark timing, which a cold engine needs to stay running.
When engine coolant temp gets above 170degF then computer switches to Closed Loop, this is the other 1/2 of the drive cycle, O2 sensors are working now and computer is using them to fine tune air/fuel mix on the fly.
This is often when CEL will come back on if something is out of range, so anywhere from 5 to 8 minutes after starting engine, depends on outside temp and driving conditions.
For some codes to stay cleared you need 2 to 5 complete "drive cycles", emission codes can take even longer.
TPS sensors have very wide parameters, because these are "Learned" values, MAF sensor is also Learned value.
This means the computer will need a few drive cycles to reLearn the new sensors parameters to reset its "out of range" values.
Last edited by RonD; 01-09-2016 at 11:09 AM.