There's an arrow under the 76.5C rating on the longest "half" of the unit, the arrow goes up. The unit restricts coolant from the head until it's warmed up, thereby not over cooling the oil and contributes to good fuel economy. Whether it's needed is up to the owner. When mine have gone bad, motor runs cooler than normal, soot goes up, fuel milage down. Damn expensive from Isuzu and I've never found another source.