How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Steve Your Own Question
Steve, Service Manager
Category: Chevy
Satisfied Customers: 5494
Experience:  25+ Years experience as a professional working automotive technician; ASE L1 master technician
Type Your Chevy Question Here...
Steve is online now
A new question is answered every 9 seconds

My 1999 GMC suburban 1500 wont start and the security ...

Customer Question

My 1999 GMC suburban 1500 wont start and the security light keeps flashing and wont stop flashing and it wont start and i was wondering how i can make it start without taking it to the dealer
Submitted: 10 years ago.
Category: Chevy
Expert:  Steve replied 10 years ago.


Your vehicle has a factory security system called "Passlock". This is a very sophisticated system that is integrated into the computer control system; therefore it cannot be bypassed or removed... This is what makes it so effective; a thief cannot simply remove or change a component and drive your vehicle away.

It works like this:

The ignition lock cylinder in your vehicle has a built in hall effect generator that sends a signal out over the class2 serial data network when the correct key is turned in the ignition. The instrument cluster (or the body control module, depending on model) receives this signal and compares it to one that was stored when the vehicle was first programmed. If the 2 signals match, the ipc sends a second "start enable" signal out over the network that is received by the powertrain controller. When this signal is received, the PCm enables fuel injector operation so the engine can start.

If the security light in your vehicle is on or flashing, this indicates that this process has not completed; it could be due to a failure at any point in the chain of events. The only way to diagnose this type of failure is with a Vetronix TEch-2 scan tool; this is the Gm factory scan tool and is the only one on the market that can access the security system to retrieve fault codes and monitor module communications. There simply is no other way to diagnose the fault without this piece of equipment.

Also, just to complicate things, if any of the components are replaced (such as the ignition switch), the tech-2 is required to program the vehicle to recognize the new part or the engine will not start.

The usual failure component in this system is the hall effect generator that is part of the lock cylinder, although internal BCM failures and poor electrical connections are seen occasionally.

When a failure occurs and the light comes on, the system locks out another starting attempt for 10 minutes. After the 3rd unsuccessful attempt, starting is locked out for 30 minutes.

Working at home without the required diagnostic equipment, about all you can do is to try every 10 minutes to get the vehicle to start; if it does not, walk away and do not touch it for another 10 minutes. If you are lucky, eventually a good key cycle will occur and the engine will start and run untill shut off (when it may not start again).

You do not have to take the vehicle to the dealer to have this diagnosed and repaired; many independent shops have a tech-2 and a technician trained in Passlock system diagnostics. It is, unfortunately, not something you can fix yourself at home in the driveway.

I hope this is helpful and answers your question; if so, please click my accept button! thanks!