If it's a V engine, one for each cylinder bank, both must be same heat range, remove thermostat housing and both thermostats, clean receptacle, replace for new, torque well, and that's it...
One is for the cylinder head and the other is for the block. As you look down on the 2 housing holes looking to the rear of the engine, the left thermostat is for the head and it has 3 raised bumps on the silver top. The right is the block thermostat and only has 2 of these bumps. They are different opening temperatures. The head is fully open at 140 degrees and the block is fully open at 155 degrees.
