Security Light On
Drifting
Joined: Jul 2006
Posts: 1,857
Likes: 106
From: Sloatsburg NY
Oldtimer
Tech Contributor
Joined: Dec 2003
Posts: 19,384
Likes: 87
From: Horncastle Lincolnshire, England
2023 C5 of the Year Finalist - Unmodified
When the BCM sends the signal to the key it looks for the resistor value programmed into the BCM. If it gets a bad value (wrong key) the security light stays on and the start is inhibited.
If it gets the correct value the theft deterrent relay is energized, the column lock is released and the PCM is told to enable fuel delivery. Then the car runs.
If and intermittent fault then occurs (say the key loses contact because it or the barrel is worn) a DTC would be set during the "cycle". The car wouldn't stop for safety reasons.
The book shows shorts in the circuit, poor connections at the BCM or gnition switch ignition lock contacts or resistor pellets dirty or loose as reasons for intermittent faults.
For that reason if the OP is getting the car going I suspect something is worn.
Tech Contributor
Joined: Dec 2003
Posts: 19,384
Likes: 87
From: Horncastle Lincolnshire, England
2023 C5 of the Year Finalist - Unmodified
Hows this for a scenario?
OP holds heavy weighted keyring in hand while car is started. Key works fine, resistor checks out OK and fuel is enabled.
Let go of stuff on keyring and weight now makes key loose in ignition. Car continues to run but security light comes on as DTC is set.
Possible fix:
Take key off keyring and use just the key to start the car. Does the Security light come on with no weight on the key?
OP holds heavy weighted keyring in hand while car is started. Key works fine, resistor checks out OK and fuel is enabled.
Let go of stuff on keyring and weight now makes key loose in ignition. Car continues to run but security light comes on as DTC is set.
Possible fix:
Take key off keyring and use just the key to start the car. Does the Security light come on with no weight on the key?
Melting Slicks
Joined: Jul 2013
Posts: 2,012
Likes: 23
From: Anaheim CA
It doesn't. But a key can be "wrong" intermittently if the resistor isn't making solid contact. And I doubt it checks the key after the car is running.
Team Owner
Joined: Jul 2006
Posts: 22,162
Likes: 4,171
From: Kannapolis NC
Read the first post, it says started the car and the security light stayed on. If it doesn't read the key after the car starts, why did the security light not go off when it allowed the car to start? He doesn't say he tried to start the car and then retried and it started and the light stayed on. Something more than a resistor value is in play here.
Le Mans Master
Joined: Sep 2013
Posts: 8,900
Likes: 1,074
From: Wisconsin
Melting Slicks
Joined: Jul 2013
Posts: 2,012
Likes: 23
From: Anaheim CA
Read the first post, it says started the car and the security light stayed on. If it doesn't read the key after the car starts, why did the security light not go off when it allowed the car to start? He doesn't say he tried to start the car and then retried and it started and the light stayed on. Something more than a resistor value is in play here.
And note that we only have one code reported out of what sounded like a lengthy list.
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Team Owner
Joined: Jul 2006
Posts: 22,162
Likes: 4,171
From: Kannapolis NC
That's ok, sounds like you have it figured out. Any other opinion would just be silly.
Burning Brakes
Joined: Jan 2008
Posts: 804
Likes: 4
I have a very similar thing happening and been ignoring it for a while, probably not wise 
My car starts and runs fine but occasionally when going over a rough patch of pavement or hard banking in some turns the security light will come on and stay on till I stop and restart the car. My key fits pretty loose in the ignition so I attribute this condition to a worn out cylinder and loss of pellet contact when key is jarred during driving. This would also suggest that the system constantly reads the key resistor value and goes into security mode anytime it is interrupted, even after the car is started. It would also suggest the car will remain running when security circuit is tripped, safety would be a good reason. It would be wise on my part to replace the cylinder.
My car starts and runs fine but occasionally when going over a rough patch of pavement or hard banking in some turns the security light will come on and stay on till I stop and restart the car. My key fits pretty loose in the ignition so I attribute this condition to a worn out cylinder and loss of pellet contact when key is jarred during driving. This would also suggest that the system constantly reads the key resistor value and goes into security mode anytime it is interrupted, even after the car is started. It would also suggest the car will remain running when security circuit is tripped, safety would be a good reason. It would be wise on my part to replace the cylinder.