How to reset 4wd light for trailblazer 2003 - 2002 Chevrolet TrailBlazer

Service 4WD light will come for a number of reasons; TCCM is not communicating, TC actuator is not following TCCM command, Front splined disconnect actuator is not engaging.

First thing to check is the condition of your battery. Don't know what these problems have to do with selecting 4WD. But if battery is weak or dead, you will have all these conditions.

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yes and no the fork is lubed by splash not submersion in gear oil also rod is only extended when in use normally not exposed for most of its life this is the most common problem with 4x4 systems the actuator rod/fork sticking 4x4 for most owners is a limited use option

actuator may need service[lubed ]connection or wiring loose or damaged

is this the check engine yellow light?

check the lubricant in the transaxle, if it is low it will cause the light to come on.

With scan tool reset the ecu, if light still on check for 4wd opertation if its ok, take the car to a specialize technitian for diagniostics.

PO172 means that an oxygen sensor in bank 1 detected a rich condition (too little oxygen in the exhaust). I would replace the thermostat first. The computer could be injecting too much fuel because it thinks it never gets to operating temperature. That may keep the light off.

There is a relay switch box under dash. But there is also a vacuum diaphram in the passenger front fender, which operates the front axle, that turns the light on and off along with the cable movement. Also there is a vacuum switch on the transfer case. When you flip the switch it sends the vacuum to the switch on transfer case which allows the vacuum back to the diaphram to pull the front axle cable. I would check for the vacuum lines first.