The red error LED has multiple meanings for Galil motion controllers.  Here is a list of reasons the error light will come on and possible solutions:

1)  Under Voltage - if the controller is not receiving enough voltage to power up.

2)  Under Current - if the power supply does not have enough current, the red LED will cycle on and off along with the green power LED.

3) Position Error - if any axis that is set up as a servo (MT command) has a position error value (TE) that exceeds the error limit (ER) - the error light will come on to signify there is an axis that has exceeded the position error limit.  Use a DP*=0 to set all encoder positions to zero or a SH (Servo Here) command to eliminate position error.

4) Invalid Firmware - if the controller is interrupted during a firmware update or an incorrect version of firmware is installed - the error light will come on.  The prompt will show up as a greater than sign ">" instead of the standard colon ":" prompt. Use GalilTools software to install the correct version of firmware to fix this problem.

5) Self Test - during the first few seconds of power up, it is normal for the red LED to turn on while it is performing a self test.  If the self test detects a problem such as corrupted memory or damaged hardware - the error light will stay on to signal a problem with the board.  To fix this problem, a Master Reset may be required.  The Master Reset will set the controller back to factory default conditions so it is recommended that all motor and I/O cables be removed for safety while performing the Master Reset.  Cables can be plugged back in after the correct settings have been loaded back to the controller (when necessary).  To perform a Master Reset - find the jumper location labeled MR or MRST on the controller and put a jumper across the two pins.  Power up with the jumper installed.  The Self-Test will take slightly longer - up to 5seconds.  After the error light shuts off, it is safe to power down and remove the Master Reset jumper.  If performing a Master Reset does not get rid of the error light, the controller may need to be sent back to the factory to be repaired.  Contact Galil for more information.