clearing check engine codes via negative battery terminal disconnect
 

I've had a service/check engine light on now for a few thousand miles. I meant to get to it but never did as she seemed to have no changes after the light came on.
Got a free code check and overall inspection from South Bay Land Rover and the service manager failed to give me the code but said I need a new purge valve (guessing it was the P0440 code?)
I need a smog test now and before buying and installing a new purge valve (dealer wants $130!- and thanks guys for the $45 Hyundai tip), I hoped to clear the code by disconnecting the negative battery cable, thereby passing smog (hopefully) without replacing the purge valve.
So I did the battery trick just now, waited about 30 seconds before reconnecting, then started her and no go- light still on.
Do I need to wait longer, or is this method not foolproof?

That method is for the OBDI systems, those are '95 and older, cars have gotten smarter that the driver now.
You need a OBDII code reader.
And no you cannot just reset the light and then pass the inspection, the system will read "not ready" and you will fail because that tells them you just reset the light.
Once you reset the system you need to drive it because the computer learns how you drive and adapts the system to give you the best all around performance for your driving style.
So you need to fix the problem and rent or buy a OBDII code reader or register your truck in a state that does not do inspections.

When you remove your battery from your laptop does all your data disappear? Fix your purge valve problem solved.

You can't get past the California test, the 440 will need to be fixed and the code cleared properly cause if they catch it, you will be failed and become a "Gross Poluter" and then things become expensive.