my impact wrench seems weak, it won't bust loose lug nuts it used to handle fine.

Diagnosis

The loss of torque in your Makita XWT15Z is typically caused by a failure in the kinetic energy transfer within the hammer-and-anvil assembly. In an impact wrench, the motor spins a heavy internal mass (the hammer) that strikes an anvil to create rotational force. When the tool feels "weak" despite the motor spinning at full speed, it usually means the hammer dogs have become rounded or the anvil has developed excessive play. This prevents the hammer from delivering a sharp, concentrated blow, causing the energy to dissipate as vibration and heat rather than torque. Another common culprit is a breakdown in the electrical current path. The XWT15Z relies on a high-burst of current to reach maximum RPMs. If the battery terminals are oxidized or if the battery cells have high internal resistance (common in older 3.0Ah packs), the motor cannot reach the critical velocity required to generate full impacting force. You can confirm this by testing the tool with a fresh, high-capacity 5.0Ah or 6.0Ah LXT battery; if the torque returns, the issue is the power source, not the tool's internals.

How to Fix It

To restore the torque of your XWT15Z, you must first eliminate power bottlenecks and then address the mechanical lubrication. Follow these steps:

1. Remove the battery and inspect the gold-plated contact pins on both the tool and the battery. Use a cotton swab dipped in 90% isopropyl alcohol to clean any carbon buildup or oxidation from the terminals.
2. Check the mode selector switch on the base of the handle. Ensure it is set to the highest torque setting (usually indicated by the largest icon) and not in "Bolt" or "Nut" mode if you are attempting to break a seized lug nut.
3. Perform a "dry-fire" test: Run the tool for 5 seconds without a socket attached. If you hear a high-pitched "whining" or a rattling sound instead of a rhythmic thumping, the internal hammer is slipping.
4. Apply a high-pressure molybdenum grease to the anvil. Since the XWT15Z is a sealed unit, you cannot easily grease the internal hammer, but lubricating the square drive end reduces friction losses. Apply a small amount of grease to the 1/2" drive square and rotate it manually to ensure it is seated properly.
5. Switch to a 6-point impact-rated socket. If you are using a 12-point socket or a chrome-plated hand socket, the energy is being absorbed by the socket's flexibility rather than the lug nut.

If That Didn't Work

• Replace the Hammer and Anvil Assembly: If the tool continues to struggle, the internal hammer dogs are likely rounded. You will need to disassemble the gear housing and replace the hammer assembly. This requires a full teardown of the planetary gear set. If you see metal shavings in the grease during disassembly, the anvil has failed and must be replaced.
• Upgrade to a High-Capacity Battery: The XWT15Z performs significantly better with 5.0Ah or 6.0Ah batteries compared to the 2.0Ah or 3.0Ah versions. The higher discharge rate allows the motor to hit peak RPMs faster, which is essential for breaking seized bolts.
• Check for Internal Gear Slippage: If the motor sounds like it is racing but the anvil isn't moving, the planetary gears may be stripped. Open the casing and inspect the teeth of the gear train for missing chunks of steel.
• Inspect the Trigger Switch: A failing trigger switch can cause "voltage sag," where the tool doesn't get the full 18V required for max torque. Use a multimeter to check if the output voltage at the motor terminals drops significantly when the trigger is pulled.

Heads Up

• Never use "cheater bars" or extensions on an impact wrench, as this changes the leverage and can snap the internal anvil of the XWT15Z.
• Avoid using the tool at maximum torque for extended periods; once the lug nut "cracks," switch to a lower setting to prevent over-stretching the bolt.
• Store your tool and batteries in a temperature-controlled environment. Extreme cold increases the viscosity of the internal grease, which can make the tool feel sluggish and weak during winter use.
• Only use genuine Makita LXT batteries. Third-party batteries often lack the peak current delivery needed for the high-torque demands of an impact wrench.