Ok, that was weird getting logged in. I forgot about some of the other threads mentioning the lower case. Sheesh... The Security Light on means there is a problem wih your theft deterrent system. Your regular mechanic is wrong about one thing. You don't have to go to a dealer to have this serviced. An independent shop that has stepped up to the plate, and purchased the correct tools and information to deal with this system can do the work just fine. I would be one such shop/technician that is equipped, trained and perfectly comfortable servicing your vehicle for this issue. As far as the system not working correctly and caring about it go, I suspect the system has recognized a failure and has shut itself down which allows you to use the car for now. But there is likely a demon lurking, and if you lose battery power for some reason you could end up with a no-start and require being towed to a shop set up to service your car. The reason this could happen is as long as a certain trouble code is in the computers memory, it goes into a fail safe mode. I suspect this fail safe trouble code is currently set, as well as the system failure code. But if you lose your "keep alive memory" in the theft deterrent, then you will lose the failsafe code, and only the system problem will remain, hence potentially the same condition as an attempted theft, and therefore a no-start. Either head to the dealer, or find a top gun independent to have this system on your car properlly repaired.

Not hard to find exactly what you described at all. Check with NAPA auto parts or any other parts store. Here is a link that might be useful: side steps

Thanks again john_g, and I enjoyed reading an old post of yours on the same subject, concerning a S-10, the model replaced by the Colorado. In any case, careful reading of the owner's manual (I usually read for speed rather than content and miss all the important stuff). From what I read I see the Passlock intervention is limited to the fuel system, disables the fuel injector(s) if there is an attempt to start the vehicle without a "proper" key in the ignition tumbler. So, why the symptom no starter crank untell the release of the key? Also, it seems the latest symptom of cranking but the engine dies as soon as the starter releases is consistent with the fuel being cut off. Looks most like a tumbler problem, i.e., a legitimate key is not recognized/registered by the tumbler (or associated electronics - I also tried a back-up key with the same result). It seems possible too that a defective tumbler could intermittently not make the contact to engage the starter until the key is released - the fact that the starter then disengages when the engine starts is a still a mystery. Guess I'll have to take it to the expert, and the Dealer seems to charge $150 and hour, so the price will be in the hundreds. Not a feel-good for Chevy in my book. My last previous Chevy was a 1969. Maybe I'll wait another 40 years before I buy another... oh, yes, I and perhaps GM will not be around in 40 years.

How hot is the transmission getting? Are you using scan data to report the actual transmission temperature? My first thought is that the transmission cooler inside the radiator might be restricted. If that's the case, and you have added an external cooler in series with the radiator, then you probably haven't fixed the problem yet. The fluid may simply not be moving fast enough to sufficiently cool the transmission. Worse yet, this cooled fluid is the main lubrication for the transmission before it goes back to the pan. That means if the fluid flow through the cooler is low, the tranny could be starving for lubrication. It takes a huge fluid to air cooler to eliminate the one in the radiator. Most coolers people buy are just to supplement the factory original in the radiator. They usually are not intended to eliminate the factory cooler. But that may be what you need in this case, or else a radiator repair/replacement. One thing you need to do is get a power flow diagram for the transmission. To get Fourth Gear, you need to see which clutches need to be applied. Then compare that to reverse, and overdrive I expect is fouirth, you just may be referring to the lock-up convertor application. When the tranny fails, what are you getting when you put it into reverse, an engine flare (free rev)? Is this what happens when it tries to shift from third to fourth as well?