I've got a 2005 Cadillac sts, which isn't recognizing the key fob. I've replaced the battery and the fob and still nothing. I know the car is on because when I pull on it the red security light lights up. I also have no access to the car besides under the hood.

Either the fob batteries are low, or the antennae that gets the signal has an issue. Find a local alarm installer / radio installer or auto locksmith. The can test and/or replace the batteries and check the system with a pro grade scan tool computer..

you can try this, with the battery connected lock the car using the key fob, make sure no keys are in the ignition. Disconnect the battery again and let it sit for 30 min or so, reconnect the battery and using the key fob unlock the car, it should start, if not repeat the procedure and insert the key into the door lock to unlock the car, both of these solutions should reset the security system. Good luck.

accsess fault memories and erase trouble codes , u need a code reader for that

Raise the car up,support it in the front

Put a battery charger on the battery cable
at the starter and the ground to the sub frame

Charge the battery for a couple hours,then try to
open the door with key fob

I had the same problem....replace the fob battery.

Check the fob battery you might need to replace it, The battery only has about a 3 year life

There should be a small slot in the storage compartment either in the passenger side or the consel, place the fob in there an see if car starts, if so the the battery in the fob needs replacment.

My cadilliac sts 2005 key fob started no working. when I try to start it the car tells me that the key fob is not detected. I just changed the battery and it is still doing it. What else could be wrong with it:

The key fob does not get charged. Instead you replace the battery inside the Fob. if you look behind the fob it tells you what battery it needs for replacement. CR2032

Slight glitch from what I've found. Has happened to me at the gas pump. One other time while talking outside my car with out opening the doors. If No FOB detected then step out of the car lock and unlock, get back in and try. If still dosen't detect then you need to walk twenty feet away with your key and it should reset.