NBS trucks have up to 3 blend door actuators (not including the recirculation/fresh air door actuator): one for the driver's side, one for the passenger side, and one for the rear of the cab on extended cab and crew cab models. If one of these actuators goes bad you will feel heat when you expect AC (or vice versa) in the location whose blend door is controlled by the actuator that's gone bad. These actuators are all the same part (15-72971), regardless of position. As of this writing, they run approximately $260ish from a dealership. They can be had online for about half that. I purchased mine for $100.43 (prime-eligible, so free 2-day shipping if you're an Amazon Prime member) from Amazon.com here: http://www.amazon.com/gp/product/B0011YHOE4. I found a really good step-by-step guide for the replacement. Instead of taking pics of my removal/replacement, today, and reinventing the wheel by posting them ... it makes more sense to cross-link the guide I found and used, as it has fantastic explanations and photos. Props to Sparky... You must first remove the dash cover. (Guide here: http://www.sparkys-answers.com/2011/03/2004-chevrolet-silverado-dash-cover.html) Once done follow this guide to remove/replace the passenger side blend door actuator: http://www.sparkys-answers.com/2011/03/2004-chevrolet-silverado-changing.html If you're familiar with the dash cover removal and comfortable working on your truck, expect this to take 3.5-4 hours, as the passenger side actuator is buried in the dash. Those for whom this is new, expect it to take 5-6 hours. The dealership, itself, quotes 4.8 hours for the passenger side actuator replacement (at 80ish bucks an hour, usually), so that'll give you some idea of what you'll save by doing it, yourself. Surreal Disclaimer: As of this writing I have a 2004 GMC Sierra. I believe these instructions are good for 99-06 Silverados/Sierras, however, there may be minor differences based on brand and/or year. I strongly recommend getting the actuator part number from a dealership and comparing before ordering based on this post, alone.