This is my first time posting (brand new member - looking to pick some brains on an issue) I know there have been a lot of threads about starting problems either caused by a dead battery, bad ecu or cps. This issue doesn't seem to fit exactly as other threads I have read so I wanted to get some input from people that might know the answer.

Background:
2003 CTS, 3.2L v6, ~80,000 miles, no warr.

Symptoms:
Two keys/key fobs for the car, started the car the other day with the first key and got a message on the DIC (driver information center), saying "check theft deterrent system". The car would start, but would not shift into gear. Tried the other key and the car started up and ran fine. Figured the chip in the first key may have been damaged from being in my pocket or something.
A few days later, was out in the morning running some errands, everything was fine. That afternoon, went to go out, got the "check theft deterrent system" message again. Shut the car off, tried to restart it, when I turned the car on the DIC came on, as did the climate control display, and all the warning lights, as usual, they then went blank and the car would not start. The check engine light stayed on but blinked on and off randomly and the car locked the key in the ignition. From outside the car you could hear a relay or something under the hood clicking on and off, seemingly randomly, disconnected the battery so it would not run down. No warning lights or anything on just that theft message with the first keys and then with the second. The car ran fine the last time I drove it before this.

I know sometimes its a bad cps but from what I have read you can usually turn the car over, it just wont start.
I have read some threads where people have had the issue while driving (I didn't but could have been just a timing issue).

I am not sure what the clicking would be under the hood, and I don't recall reading any threads that mentioned that (could have just not heard it).

I am leaning towards a bad ECU but before I start dishing out a few hundred $$$ I would like to see if anyone else has some ideas or suggestions.

Thanks for the help!

See if there are any codes on the DIC, hold the F1 and F6 for 3 sec and use the tuning dial to read the codes. The ones that end in C are current, H = History.

BTW, hold old is the battery? if it's the original, good or bad, do yourself a favor and replace it. Weak batteries cause intermittent problems with the computers on this car.

first off, welcome to the forums...

sorry to hear this about your car and i have no input, but i am very intrigued by your cars symptons. i have had some similiar symtons, but have yet to find a fix for this...if you figure something out, please keep us updated

Are you sure the battery is in good condition?
Sounds like a voltage problem on start up to me.
Once you sure the battery is good maybe we can think of some other things.

Thanks for the input, I know this topic has been asked a lot but this didn't quite seem to fit the others (however I am sure it is still probably the same in the end).

I never thought about the Battery being the issue, I assumed because it has always started the vehicle and didn't show any issues (accept for these two times) that it was fine. I will see about getting a new battery or at least getting the battery in to be tested (it is disconnected right now for fear of that relay or clicking under the hood training the battery). I will also check to see if there are any codes showing, I would like to narrow it down before I start shelling money out.

I assume I am correct in my thinking that if the battery is fine (or replaced) that the issue is likely the ECU? It seems to fit most with other similar issues, the only part that doesn't is the relay or clicking under the hood, but that could be the started still trying to engage because the key wont remove.

Hopefully there is an error code that explains it all.

Thanks.

Sounds like the battery to me. My car was working just fine. Then all of a sudden it wouldn't start, key got stuck in ignition. Lights came in the dash came on and so on. Recharged the battery and the car worked. After a week in the garage same procedure again. Got a warning light about theft.

Drove to the dealer, they checked the battery and the result was unstable. Got a new battery and the problem is fixed.(touch wood)

What I did noticed was that with the bad battery the dash display read 12,5 V when it wouldn't start, and ca. 13,5 V when it did start, also it read Battery OK. After i changed the battery I get a 14,6 V reading. The reason the car probably gave inn now is a battery at 0 Deg celsius has only 20 % of the power of a battery at 25 Deg.

Erik

That makes sense with the temp change, never thought about that but the battery is not that old, but that doesn't mean it isn't a her dud, hopefully that is all it is.
I assume if its not this that it probably is the ECM like the other cases and I need to pick up a new one and have the dealer program it?
I really appreciate the help.