do you mean the speed of the fan? if so, it'a relay generaly near the motor of the fan. If it's the heat who only came at high speed, you need to add some antifreeze.
The 2003 Chevrolet Express has a 4-speed automatic.
The 2003 Chevrolet Malibu has a 4-speed automatic.
The 2003 Chevrolet Corvette has a 4-speed automatic.
The 2003 Chevrolet Impala has a 4-speed automatic.
The 2003 Chevrolet Tahoe has a 4-speed automatic.
The 2003 Chevrolet Suburban has a 4-speed automatic.
The 2003 Chevrolet Tracker 2.0 liter 4 cylinder must have a timing CHAIN The Gates website does not list a timing belt for it
The 2003 Chevrolet Silverado-1500 has a 5-speed manual.
The 2003 Chevrolet Silverado-2500HD has a 5-speed manual.
The 2003 Chevrolet Express-Cargo has a 4-speed automatic.
There is no such vehicle as a GMC Tracker. The Tracker is a Chevrolet. The firing order is 1-6-5-4-3-2.
It is located in the transmission.