If the optical select-fire sensor on the MTW Spartan Electronics Control Board is giving inconsistent results, it may be necessary to clean the contacts on the board. After removing the electronics from your MTW, remove the electrical tape from the back of the board to reveal the sensor contacts. Clean them thoroughly with 70% or better isopropyl alcohol. After allowing the contacts to dry, re-apply the electrical tape and reinstall the board in your MTW.
For instructions on removing/installing the control board, see this video.