I'm having the same issues. I checked the codes and P1631 and B2723 are showing. Both keys are affected. Will the learning process have to be done with both keys or just one?

 

You don't need to do the learning process .... it is only needed if the BCM is re-programmed or replaced.

The B2723 is causing the PCM to set the P1631 ... once you fix the B2723 the other will disappear.

You need to take both keys and clean the contacts on the pellets. Use a pencil eraser to burnish the metal contacts, then use alcohol or electrical contact cleaner to thouroughly clean the contacts. Insert a key without a key chain (those 28 pound keychains some people put all their keys on can add to this problem) and pull it back out ... add contact cleaner/alcohol and re-insert. Remove key and wipe off. Add more cleaner and re-insert. Do this several times as you are trying to clean the contacts within the lock cylinder.

Then try starting the car. Remember that if it won't start, there is a 3 minute "time-out" whenever an "invalid" key is used.

If the problem persists you are looking at replacing the ignition lock.

 

Once knee bolster is off it will expose your ignition, you will note the 2 white wires coming out of the front of it, follow it down to the connector and unplug, cut the wires on the ignition side insert the correct value resistor with shortened wires and srink wrap, soldier it in, recheck your resistance to ensure you havn't burnt out the resistor, plug it in and it will start, see attached pics

cost about 5 cents by cutting the wires as suggested you can replace your ignition at any time, they cost about $75

 

Thank you both for the info. I'll try the cleaning process first.

 

 

Tried the cleaning process but it's still shows the "security" lite and periodically wont start. If I replace the ignition switch will I need new keys?

If so what about my door locks as they would be the original keys?

Thanks for your help!

 

[QUOTE=CJR2;1566744670]Tried the cleaning process but it's still shows the "security" lite and periodically wont start. If I replace the ignition switch will I need new keys?

If so what about my door locks as they would be the original keys?QUOTE]

Actually, you will have the new ignition cylinder cut to match your current key (any locksmith should be able to do it). Therefore, you don't have to worry about your door locks!

 

 

tried the resistor and no start. How do i know if its the ignition switch?

 

\
how do i find this diagram?

 

Search "car wont start"

Narrow your "advanced search" parameters to:
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[QUOTE=RedRider98;1566744930]The “key lock cylinder" & "VATS contact", & the "electrical switch assembly", are separate parts of the “ignition switch” and can be replaced independently.

 

4.49 ohms across key pellet & 4.66 on 2nd key. This isn't in line with the +/- 10% values. Can this be a no start issue which I am having once in awhile?? The car starts up 98% of the time with theses keys though.

 

Well my problem just started happening recently. I noticed the security light stays on while driving and wont go off. I didnt take notice to it the 1st time. But the next morning when i went to try and start my car it wouldnt start. Everything would power but nothing would start. I disconnected battery and nothing. I then tried to lock and unlock and that ended up doing the trick. Every other start up the security light stays on. And the no start has only happen once since the other morning. But i just did the lock and unlock and it powers right up. What gives?

 

great post. I will add this to my important C5 post list!

 

Nice write-up.

 

Hello together. My name is Roger. I'm new here in the forum. I have since 2 years, a 2000s C5. For some time I have the problem, approximately one in ten fails to start attempt. I then have to wait 3 minutes until I can start the engine. I've cleaned the contacts on the key that does not help. The second key behaves the same. Can the problem may also have to do with the ignition switch on. For information: I also get the error code B2721 (PASS-Key Detection Circuit) read from the error memory.
Thanks for your help. Regards, Roger

 

Roger

WOW. First time that I have ever helped someone from Switzerland.

I sent you a PM.

 

well now with both lower halves of my dash apart here is where I am:
had intermittent no start- fortunatelly now it is always no start-

NO codes repeat no codes

changed starter and solenoid - no crank
VATS bypass with resistor- no crank
new ignition switch- no crank
replaced TDR - no crank
new keys- no crank
VATS relearn just for kicks- no crank
checked/cleaned grounds - no crank
checked PCM/BCM wiring and connections - no crank

BUTTT- if I, with key ON

manually jump across starter solenoid to crank motor- start and run fine
manually jump across purple and red wired at TDR to crank motor- starts and runs fine
and i think this is the most important- if i provide a direct ground from the black and yellow wire at the TDR - starts and runs every time

i have a feeling the BCM for whatever reason is NOT giving me the required ground at the TDR during cranking- when i test for ohmage to ground via multimeter with key on cranking/starting position i get infinite resistance

sooooo- if i hardwire this bypass any ideas what can/will happen? another thread poster said at some point the VATS will kick in and i will have no fuel/etc and the car wont run

ps i have checked all the fuses

this is an Arizona garaged car its whole life- dont laugh- no signs of wiring being chewed- we do get that out here

i'll owe you not a case of beer on this one- but a keg!

oh I did NOT check the auto transmission neutral safety swith- looked for threads on this everywhere and could not find any such as even where the thing is- i assume it is at the shifter
thnaks in advance!!!!!! George
1998 C5 convertible/stock except for Mich run flats and Hawk ceramic pads

 

Do they make a keg of wine???

How are you reading the DTCs????

Is your SECURITY LIGHT in the IPC on or flashing??

The Yellow/Black wire comes directly from the BCM and the BCM will only complete that ground path when all the cars security measures are satisfied. If something like VATS is not correct, it will NOT allow the BCM to provide that wire a ground.

Have you tried to disconnect the battery and reconnect it and then tried to start the engine? Try that.

Next

I know you stated that the car doesnt get wet BUT, if you run the AC you could have or have had moisture on the BCM. If the HVAC drains clogs it will drip inside and wet the BCM.
You can remove the BCM and remove the circuit board and inspect it for a white corrosion that can form on the bottom of the board... Also check the board for any obvious damage.

Is the wire making the circuit to the BCM? Read it out at the BCM connector.

Is the BCM functioning correctly???

If you ground the yellow/black wire and the car starts when you do all the NORMAL start stuff to start it,,,,, all the other stuff in that circuit is GOOD!

Lots of other things to check to pin down the issue.

Keep me informed on your findings.

Bill