Thursday, January 30, 2014

2004 Yukon XL (Suburban, Silverado, Tahoe,Sierra, etc) "Reduced Engine Power"

Solving "Reduced Engine Power" on a 2004 Yukon XL (Suburban)
(also applies to Suburban, Silverado, Tahoe, Sierra, and Yukon models plus 2005, 2006 model years)

Last year while driving our 2004 Yukon XL out of town on a Saturday evening, the truck suddenly dropped to idle, displayed "Reduced Engine Power" and would not respond to any accelerator pedal input.  We were able to idle down a few side streets and into an auto parts store parking lot.  After checking the truck with a scan tool they told us that the computer was bad and we needed a new one for $300.  I doubted that was really the right repair so I spent a few minutes using my phone to Google around and soon found that disconnecting the battery for 10 minutes or so would reset the fault, hopefully letting me drive far enough to get a hotel for the night and buying me enough time to figure out the real problem.  I popped the hood and started to disconnect the negative wire from the battery but immediately noticed that it was loose and barely attached.  I went ahead and took it off, left it off 10 minutes, securely reconnected it, and drove off without another problem for nearly a year...

------> Problem solved in 45 minutes for $0 by tightening up the negative battery connection.

Which brings me to this week.  The truck had been sitting in the driveway for several weeks since it was last used and as soon as I started it the "Reduced Engine Power" message reappeared.  Again there was no response to pushing the accelerator pedal.  I felt pretty confident that I knew what was going on so I popped the hood, disconnected the battery, cleaned up the little bit of corrosion on the terminals, waited 15 minutes, reconnected everything, and started it back up.
Same problem.  Back to Google.
I found a number of references to cleaning the throttle body in order to solve this problem so I dumped a can of carb cleaner into the throttle body (I just pulled the intake air hose off and manually rotated the butterfly valve out of the way), reconnected the hose, and restarted the engine.
Same problem.  Back to Google.
I found another report from a guy who had found that the throttle control box was positioned on the firewall where it gets wet and water related malfunctions were causing his issue.  So I decided to look at that box and see if it was wet.  Well it wasn't wet but I did happen to notice that the wiring harness leading up to the throttle controller was literally chewed through.
16 wires, split conduit, electrical tape - completely severed by some mischievous critter.  So, after running to Radio Shack for some 22 gage wire and shrink wrap I was ready to make the repair.  Now the problem was that there are 16 wires and only 14 different colors - there are two black wires and two gray wires in the connector!  This is where the AutoZone website came to the rescue.  They have wiring diagrams available online for free if you sign up for an account.  Using the wiring diagram I found that one of the black and one of the gray wires go to the throttle body connector.  I unplugged the connector off the side of the throttle body, found the gray and black wires and used my vohm meter to determine which black and which gray wires in the harness went to the throttle body and therefore should be spliced to pin 2 and 3 on the throttle control connector.

With that problem solved, all I had to do was cut sixteen jumper wires, solder them into the harness, shrink wrap and tape it up.  After all the splicing was done the truck fired right up and runs like normal.

--------> Problem fixed in 4 hours for $12 ($4 wasted on carb cleaner $8 for wire and shrink wrap)

While it is seems unlikely that your "Reduced Engine Power" message is due to a wiring harness that has been chewed in half, you never know till you check!