Check impact wrenches regularly to be sure that what you tighten stays clamped in place. The Technology & Maintenance Council (TMC) of the American Trucking Association, in Recommended Practice No. 222A, says, "A reduction in torque can result in loose wheel nuts, cracked wheels, and wheel separations."
Simply install a wheel nut on a vehicle using the air tool. The wrench should impact for 3 to 5 seconds. Then measure the torque on the nut using a torque wrench in the tightening direction. Another option is to send the wrench out to a service center to be checked for torque and restored to its original performance. Of course, if you have a Skidmore-Wilhelm gauge, it will measure impact-wrench performance.
If the air wrench won't achieve working-range torque, it may need to be rebuilt. But first make sure it's getting enough air.
Most air tools require 90 psi of running pressure to operate properly. Measure air-system pressure at the tool by plumbing a pressure gauge into the air line at the tool drop. Check the pressure when the tool is running under load, and make sure other common drains on the air system — such as other impact wrenches, lifts, and pumps — remain in use.
Source: TMC RP 222A