Reported Issue: Bought used car with a Code Alarm system installed (can also apply to most other brands of systems).
Situation: Remotes lock and unlock vehicle but no crank or power to starter motor. Replaced battery in vehicle.
Possible Cause: Internal or external starter defeat relay stuck open (very, very rare possibility, especially on system's with external starter defeat relay).
Possible Fix: On system's with external relay: Press unlock (or disarm) on remote: Hold relay in hand and turn ignition ON, if you feel the relay click then it's a bad output from the system: Commonly you'll find a Solid ORANGE wire connected to the relay. Cut this orange wire in half to permanently disable the starter defeat relay.
If the relay doesn't click, this is normal operation and indicates that the system may not be causing the no start problem. To double check that the relay is working properly, close all the vehicle's doors, press lock (arm) on the remote. Wait at least 30 seconds and again holding the relay in hand, turn the ignition on, the relay will click ON. Then turn ignition off and press unlock (disarm) on remote. The relay will click once more (OFF).
Systems with internal starter defeat relay: In general, most systems will have 2 wires that are the same color and gauge, and the wires will be running into the same plug commonly right next to each other. One of the wire's may have a color stripe on them. On most Code Alarm system's for example, these 2 wires will be 2 purple wires (1 with a red stripe) or 2 white wires (1 with a red stripe) or 2 yellow wires (1 with a red stripe). You will have to place a jumper wire across these two wires to connect them together. You cannot just disconnect the module! If it's the system causing the no start problem then the vehicle will start right up after jumping the wires together.
When the above procedures have been applied to your specific system and the vehicle will still not crank, it's definitely a mechanical or electrical problem with the vehicle and not the system causing the problem.