2003 yukon no crank when it's cold out

everything said so far is great info, but you didn't tell us if it's set for voltage sense or if you used the tach wire. it may not be that complicated. vipers are great for programming a stubborn car to run on voltage check and get them to work, but of course tach is always better. if we had this info it'd be easiewr to find out what's wrong. even if you did hook up tach then at least there's no further questions and we know it's passlock kicking you out.

sparkie wrote: If the engine won't turn over, check the battery terminals are clean and tight. Take the terminals off and inspect the rubber boots aren't hiding corrosion. Clean the main battery ground cable at the frame rail above the driver's side tow hook behind the bumper. Check that the ignition switch and other dash harnesses are fully plugged in. Proper diagnosis requires detailed info of what is going on in order to fix the problem.

we can rule those out sparkie. if it were any of those problems, it wouldnt start with the key either.

It still may be a Passlock problem - on some vehicles if the BCM receives an incorrect R-code it will cause a "crank, no start" condition. On others it will cause a "no crank" condition. ernmang2, I would suggest the following test, assuming you've got a good battery. 1. Go to the underhood fusebox, and remove the "Starter" relay. 2. Turn the key to the "Run" position(where it would be while driving). 3. Using a jumper wire, short together terminals 87 and 30 where the relay was plugged in. If it doesn't crank at all, check the "Ign A" 40A fuse in that fusebox. If the fuse is good, try shorting 12V from the battery directly to terminal 87. If it still doesn't crank it's most likely a problem with the starter itself. If it cranks and runs, that suggests a problem with the starting circuit on the vehicle. Check the "Crank" 10A fuse in the underhood fusebox to see that it's not blown and has 12V during crank. If it doesn't have 12V check the yellow starter wire at the ignition switch for a break. If it cranks, starts, and immediately dies, that suggests a problem with the Passlock. Verify that the bypass module is plugged in and that the 3 wires from the Passlock sensor in the key cylinder are not broken. (The yellow R-code wire may have been cut and connected to 2 separate wires from the bypass module, depending on the particular bypass used. Verify that these connections are intact.C Renner's Auto Electronix

My service is cheap, quick, and good - pick any two

The 1st thing is to see if the security light goes off Put the key in and DONT start it just turn it to IGN Not RUN wait for bout 4 secs and if the security light goes off, it is not the IGN or resitors in it If the light stays on its the Bypass or IGN