Volo VP12 Performance Chip/Power Programmer for 2004 GMC Envoy 4.2L 5.3L This high-speed microprocessor soft-flashes your Envoy's ECU to produce real world gains of 10-20HP and 2-4MPG. Installs behind the OBD-II port in minutes. Installation video and vehicle specific wiring print-outs are available on our website. The Volo Chip replaces timing and fuel delivery values stored in the ECU's temporary memory with optimized values. This produces the same results as expensive ECU upgrades, for a fraction of the cost. The Volo Chip DOES NOT alter rev and speed limiters. No fine-tuning or vehicle knowledge required. Just install, hit the reset button for automatic calibration, and forget it. Comes pre-programmed with maps for GMC 4.2L and 5.3L engines. Makes no permanent changes to your Envoy's computer. Will not void your GMC factory warranty and is emissions safe. Best of all, the Volo Chip is 100% guaranteed or your money back! Free firmware updates for life included with purchase.