Unplugging the batteries clears the memory.

Your car's PCM doesn't use a hard drive, it has flash memory, both RAM and ROM. The program is stored in ROM, which is why the truck will start after losing power. The learned parameters are stored in RAM. Taking the power from RAM will erase the contents.

It won't do much for a hard code - if there is an ongoing fault, the code and MIL will return as soon as the check is run.

 

never said it uses a hard drive. i was giving an analogy. it uses flash memory like a flash drive. does your flash drive forget everything when you unplug it? it does not have batteries in it....

modern vehicles (late 90's and newer) do not lose code memory anymore. the only thing it does lose when you unplug the batteries for 30 mins is your APPS calibration.

like my truck for example...I had a code once I threw by not hooking up something right and having the batteries unplugged for a long time did nothing to clear that code. first time I plugged my scanner in and told it to clear though cleared the code and it didn't come back after that...

 

i have unplugged the batteries to clear a code and it did work. once. i also noticed that the ignition will turn on by itself. just the dummy lights and it will ding at me. that started after they flashed my ECM. never did that before. i think they did mess my truck up when they flashed it

 

I'm sorry man, but you're wrong on that. There is RAM, and there is ROM. The program is stored in ROM, the learned values are stored in RAM.

Perhaps your truck is an anomaly. I'm a Subaru trained/certified mechanic, and I promise you that modern vehicles don't retain codes after disconnecting (all) batteries and bleeding off residual voltage.