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FANUC Controller
Program/G-code · Intermediate · 30-60 minutes
Fanuc Alarm 251 - TOOL CHANGE ILLEGAL T COMMAND
Alarm 251 occurs when the CNC detects an invalid or out-of-range T-code specified during an M06 tool change command. The T-code either exceeds the maximum tool number defined in the tool management parameters, references a non-existent tool pocket, or violates the tool number format defined in the system configuration.
251
Fanuc Alarm 251 - TOOL CHANGE ILLEGAL T COMMAND
SYSTEM ALARM 251
Fanuc alarm 251: SYSTEM ALARM 251. This alarm indicates a fault condition detected by the Fanuc control. Refer to the alarm number and name to identify the specific system or function involved.
Root Cause Summary
Alarm 251 occurs when the CNC detects an invalid or out-of-range T-code specified during an M06 tool change command. The T-code either exceeds the maximum tool number defined in the tool management parameters, references a non-existent tool pocket, or violates the tool number format defined in the system configuration.
Causes and Fixes
- 1
Navigate to SYSTEM > PARAM screen and check parameter 13220 (maximum tool number). Verify the T-code in your program does not exceed this value.
- 2
Check parameters 13201 (number of tool pockets) and 13210 (tool magazine type). Confirm the T-code falls within the physical magazine capacity.
- 3
Review the NC program line causing the alarm. If T-code is valid, increase parameter 13220 to accommodate the required tool number range.
Changing parameter 13220 requires parameter write enable (PWE) bit 0 in parameter 3202 set to 1. Power cycle required after modification.
- 4
Verify tool data registration in OFFSET SETTING > TOOL screen. Ensure the T-code has a corresponding tool registration entry with valid geometry offsets.
Call a technician if:
- ·ATC hardware malfunction suspected
- ·Parameter corruption requiring backup restoration
- ·Physical magazine position sensor issues
Prevention
- Validate T codes against magazine capacity before program release
- Maintain updated tool offset tables for all magazine positions
- Regular backup of ATC parameters
- Use consistent T code numbering system
Common Mistakes
- Using T00 when not supported
- Exceeding magazine capacity in programs
- Forgetting to register tools in offset table
- Copying programs between machines with different magazine sizes
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