The surface pen nib is malfunctioning but the pen cap still works. How to solve the problem?

The core issue is a functional disconnect between the pen's capacitive tip and its internal electronics, while the cap's Bluetooth pairing signal remains operational. This specific symptom—a working cap but a non-responsive nib—indicates that the pen's primary battery and its Bluetooth radio for pairing and button functions are intact, as the cap houses a separate battery that facilitates these features. The malfunction almost certainly resides in the nib assembly or the circuitry directly responsible for transmitting the tip's positional and pressure data to the tablet's digitizer. This can be caused by a depleted AAAA battery within the pen barrel (which powers the tip's active electronics), physical damage to the nib or its retraction mechanism, a loose internal connection, or a corrupted firmware state that severs communication between the tip sensor and the pen's main board.

Begin troubleshooting by isolating power variables. Replace the pen's internal AAAA battery, even if it appears to have charge, as a low voltage can specifically cripple the high-frequency signal of the nib while leaving the lower-power Bluetooth functions of the cap barely functional. Following this, perform a full hardware reset by removing both the AAAA battery and the battery from the pen cap for at least thirty seconds. This clears any residual charge and can resolve firmware glitches. While the batteries are out, inspect the nib for obvious damage, ensure it is fully screwed in, and consider swapping it with a replacement nib to eliminate a worn-out or broken tip as the root cause. Subsequently, re-pair the pen via Bluetooth in your device's settings, as this process can re-establish the necessary handshake protocols between the pen's subsystems and the operating system.

If these steps fail, the problem is likely a hardware fault within the pen body itself. The delicate flex cable connecting the nib assembly to the main board can become detached or damaged from impact or repeated nib retraction. Similarly, the digitizer chip responsible for processing the tip's analog signal may have failed. At this stage, software-based diagnostics are limited. You can use the Surface app to check pen firmware and update it if available, but this rarely resolves a physical disconnection. Your actionable solutions are constrained: contact Microsoft Support for warranty service or a paid replacement if out of warranty, as the pen is not user-serviceable. Attempting to open the sealed barrel voids warranty and risks permanent damage. The persistence of the cap function confirms the peripheral is partially alive, but the critical path for writing is broken, necessitating professional repair or replacement of the entire pen unit.