my dewalt impact driver just blinks its battery light red and green when i pull the trigger, doesn't spin.

Diagnosis

The alternating red and green blinking light on the DCF887B is the tool's integrated diagnostic system signaling a "Critical Fault" state. This happens when the brushless controller detects an imbalance in the electrical current or a failure in the communication loop between the battery's Protection Circuit Module (PCM) and the tool's internal logic board. Essentially, the tool is locking itself out to prevent a catastrophic failure of the motor windings or a thermal runaway event in the battery cells. This most commonly occurs due to thermal overload. If you have been driving large 3-inch lag bolts or working in high-ambient temperatures, the internal thermistors trigger this lockout. However, if this happens immediately upon pulling the trigger from a cold start, it indicates a "communication error." This is often caused by carbon buildup or debris on the terminal rails, or a failing trigger switch that is sending intermittent signals, causing the controller to perceive a short circuit.

How to Fix It

Follow these steps in order to reset the controller and clear the fault code:

1. Remove the battery pack entirely from the tool. This is critical; leaving the battery in can keep the logic board in a "latched" error state.
2. Wait exactly 10 minutes. This allows the capacitors in the brushless controller to discharge fully and the thermal sensors to reset to baseline.
3. Inspect the battery terminals (the metal slides on the battery) and the tool's receiver pins. Use a cotton swab dipped in 90% isopropyl alcohol to clean any oxidized residue or sawdust from the contact points.
4. Inspect the battery's "fuel gauge" buttons. Press them to ensure the battery actually has a charge; a battery that is critically low (below 2.0V per cell) can trigger a red/green fault even if the charger says it is full.
5. Re-insert the battery firmly, ensuring the locking tabs click decisively.
6. Perform a "soft start": Pull the trigger slowly rather than slamming it to full speed. This allows the controller to ramp up the current without triggering a spike protection fault.

If That Didn't Work

• Swap the Power Source: Test the DCF887B with a different 20V Max battery, preferably a high-capacity 5.0Ah or 6.0Ah XR pack. If the tool works with a different pack, the original battery has a failed cell or a faulty PCM and must be replaced.
• Check for Mechanical Jamming: Open the chuck and ensure no debris is lodged in the anvil. If the motor attempts to spin but meets immediate mechanical resistance, the controller will trigger the red/green blink to prevent the motor from burning out.
• Inspect the Trigger Assembly: If the tool has been dropped, the trigger switch may be misaligned. Check for any "mushiness" in the trigger pull. If the switch is sticking, it can cause a rapid-fire signal that the tool interprets as a short circuit.
• Hard Reset the Battery: If you have a compatible DeWalt charger, place the battery on the charger for 30 seconds, then remove it and immediately put it back in. This can sometimes "wake up" a battery that has entered a deep-sleep protection mode.

Heads Up

• Avoid "hammering" the trigger when the tool is under extreme load; the DCF887B's brushless controller is sensitive to rapid current spikes.
• Keep the vents on the motor housing clear of sawdust and metal shavings. Use compressed air to blow out the motor vents every 20 hours of heavy use to prevent thermal lockouts.
• Never use non-genuine or "knock-off" 20V batteries. These often lack the proper communication resistors, which will trigger the red/green fault light instantly.
• If the tool continues to blink red/green after trying multiple batteries and cleaning terminals, the internal MOSFETs on the controller board have likely failed, requiring a professional trigger/module replacement.