Like a lot of Chevy vehicles of this vintage, the Tahoe would occasionally not start with a flashing “Security” light on the dash. This is due to the Passlock system going bad.
Passlock is a system designed to prevent the ignition system from working if the ignition key column is forced to turn without a key. It does this with a sensor that the system uses to “learn” your keys. Most sources on the Internet advise replacing the sensor with a resistor to effectively bypass the system. However this makes the vehicle easy to steal. In my Tahoe I removed a comical rig of resistors strung together with wires and electrical tape that did this bypass since I wanted to restore the system to its original working state.
I did this but soon experienced the dreaded flashing “Security” light which meant I had to wait 10 minutes with the car ignition in the run position before I could start the Tahoe. Sometimes I had to do this once, other times I had to do it three times.
Another thing that happened was the Security light would come on and stay lit while driving. Checking the codes I found: P1631 “Anti-Theft DEvice Power Control Module / Instrument Panel Cluster Password.” I took the ignition column apart and cleaned the sensor carefully, after seeing this advised on the Internet, to no avail. I then found this website that said the Passlock Control Module on these vehicles is prone to going bad and that they rebuild them to work. It might be bullshit but I thought I would give it a try for $150.
Switching out the module was a bear but I found this great post that showed diagrams which helped a great deal. To get to the module I removed the skirt under the dash and the panel containing the lighter and ashtray. The module has tabs that hold it in place that are hard to get to, but I found I could just wedge the module loose with a screwdriver accessed through the hole left by the ashtray panel without doing any damage. It took a lot of feeling and patience to get the cable unplugged and then re-plugged into the new module.
Note: You don’t need to remove the instrument panel and access the radio chubby to get to this module.
Now that it’s in I’m hoping it will work as intended. I will update this post if it fails again.