04 LB7 won't full flash and $0537 error code

[04DMAXX]

My auto cal won't flash my ECM it keeps saying wait 10 seconds and then flickers saying trying to connect, it does this for about a minute and then I get a $0537 code. Also my security light is on but the truck still runs and drives like normal. And another thing is that whatever caused my security light to come on it locked in my old PPE tuner which I can't get back to stock so I'm trying to overwrite it. I need some help on this please!

[cindy@efilive]

From our error code list.

**
$0537 Controller is locked with a custom key.

An attempt was made to read or program a controller that cannot be unlocked using the standard GM keys.

If you know the key that was used to lock the controller, please use the unlock option in the EFILive software to unlock the controller before attempting to read or re-flash the controller again.
**

I would suggest that a previous programmer used a custom key to lock your ECM to protect their tuning information. You'll need that to unlock your ECM for tuning.

Cheers
Cindy