4x4 issue on 2003

Hey guys,

I'm having some trouble with my 4x4 (03 TRD). It doesn't seem to want to engage always - occasionally I put the knob in 4H and the little light on the dash doesn't come on. I'd say 75% of the time it switches in and I hear the click. If I start driving, 50% of the time after a couple seconds I hear another click and the light goes out (and I can confirm the front wheels aren't pulling). I've even had a couple times where it works fine for 10+ minutes, then all of a sudden I notice the light turn off. Not sure what's up - same issue if I put it in 4L. Any thoughts how I can begin diagnosing this?

A Little History (in case relevant)
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-We've just gotten a cold snap (from +16C to now -16C in a day, now still in the -15'C range). Maybe related to the cold?
-My heater core just went in the truck last week (before the cold). I heard gurgling in the morning, then on the way home I had the sickly sweet smell when using the heater... (I have not attempted to fix this yet)

Thanks!!

 

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4x4 actuator is going out. Easy fix most of the time. It wouldn't have anything to do with your heater core I doubt.