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The Dremel 4000 does not respond when the power is switched on.
While there could be several reasons for the tool to not power on, a common and simple problem is to check that the switch lever is properly engaging the electronic assembly. A guide on how to replace the electrical assembly can be found here. Dremel 4000 Electrical Assembly Replacement
In order to determine if the power cord is faulty, you will need to disconnect it from the electronic assembly and connect it to a voltmeter. A guide to explain how to replace the power cord can be found here. Dremel 4000 Power Cord Replacement
If the tool will not power on, and the switch lever and power cord have been checked for integrity, then the electronic assembly must be replaced. While there are multiple parts within the electronic assembly, the entire assembly is delivered as one piece and must be replaced in its entirety. Dremel 4000 Electrical Assembly Replacement
The Dremel 4000 will power up, but will not vary in speed.
If the tool will power on, but remains fixed at one speed despite adjusting the dial, then the electronic assembly must be replaced. While there are multiple parts within the electronic assembly, the entire assembly is delivered as one piece and must be replaced in its entirety. Dremel 4000 Electrical Assembly Replacement
The Dremel 4000 shaft will not remain locked when installing/removing bits.
If the shaft rotates even when the blue shaft lock is pressed, the button will need to be replaced. A guide to explain how this is done can be found here. Dremel 4000 Shaft Lock Pin Replacement
The Dremel 4000 makes a whistling or squealing sound when running.
If the rotary tool is making unusual sounds when running, and those sounds intensify when a load is applied, you may have faulty bearings. For this tool, the entire armature and bearing assembly is replaced at once. A guide to demonstrate how this is done can be found here. Dremel 4000 Bearing Assembly Replacement
The Dremel 4000 is overheating under standard operating conditions.
Check the four vents on the tool to ensure that they are not covered or otherwise blocked. If the vents are not covered up, but overheating persists, it is possible that dust has accumulated inside the case and must be cleaned out. A guide on opening the casing can be found here. Dremel 4000 Back Case Replacement
If the Dremel is overheating, yet the vents are clear, check that the fan on the bearing assembly is functioning. If not, the bearing assembly will need to be replaced. For this tool, the entire armature and bearing assembly is replaced at once. A guide to demonstrate how this is done can be found here. Dremel 4000 Bearing Assembly Replacement
