Ok I have tried searching for both of these things coming on at the same time and could not find anything so I'm asking for some help. About a month ago I was driving around and the ABS light came on, and a few miles down the road the 4WD light came on. The truck is not even a 4X4 so I have no idea why this would come on. I have read a lil bit about the ABS light coming on and think it may be the ABS sensors after speaking with Wheatley about the gear ratio and transmission sensor thingy, so I'm assuming its not that the tranny and tires are sensing different speeds. Also every once in awhile the parking brake light will come on, but then it goes off. Sometimes the parking brake light will flash with the ABS light, and the brake light will go out but the ABS light will stay on. So I guess I'm asking if all three of these lights are tied into each other or if I have several problems. The only codes I have run are with OBDII and its not showing any codes, so I'm asking if anyone else has run into the same problem. The truck is a 2002 GMC Sierra with 4.8L all stock with Denali rims. Thanks in advance for all the help.

 

A generic OBDII scanner will not read ABS codes, you'll either need a GM TechII or other form of scanner that has that capability. If your truck was 4x4, I could see how an ABS malfunction could affect the 4wd system, or how and ABS malfunction may cancel communication to the BCM tripping a 4x4 code. Since you're not 4x4, I'm going to assume that there's a possible glitch in the BCM logic giving you the 4x4 lamp. First thing you have to do though is get the codes scanned so you have a starting point

 

My parking brake light and ABS light come on all the time. My sensor is screwy. When the lights are off, my ABS works fine. When the lights are on, I can lock the brakes up.

 

I got my problem fixed. It was a grounding issue with the ABS module that is on the frame rail under the driver seat. You might check there and all the wiring to make sure you aren't shorting out somewhere. My ABS wasn't working at all until i fixed the short.