Mine has no high idle light? Like said above it must be in park, or parking brake on for standard. Mine starts, then in exactly 2min. the Idle picks up and the exhaust valve closes. You can really hear the difference, sounds like a jet engine warming up. If I touch anything driving related in the truck, like the brake pedal, the rpm drops immediately and the valve opens. This is because the computer thinks your going to drive, and shuts the system down. So try just starting it, don't touch nothing, get out and wait for 5 minutes to see what happens. Are you plugging in at night. Mine doesn't come on when I plug in as its too warm to start the system. I do live in Canada and its cold hear, so I assume if your plugging in then it won't go. If all fails, then I might check the temp. sensors. Most likely oil temp. Is your temp gauge working?I noticed that even when I have the parking brake engaged the light on the dash doesn't come on. So I guess I'll start checking there.
Primarily the engine oil temp (EOT) sensor. If EOT is less than about 122° F. and the intake air temp (IAT) says it's cold out, then the engine idle RPM will elevate after a coupla minutes if idling. How much the RPM elevates depends on how cold it is. Max is 900 for '99s and 1,300 RPM for 2000-up models when it's really cold.What sensors tell the computer to high idle when its cold.
If all the conditions are met (automagic tranny in park or E-brake mashed if 6-speed handshaker), fat feet not touching anything) that could be either the EOT or IAT sensors, but you should also have an idiot light if one of those sensors is on the blink.Mine hasn't high idled all winter, no matter how cold it is.