“Won’t start” sounds broad, but the failure can be reduced quickly by separating readiness, command, logic, output, energy, and motion.
SURVIVAL RULE: Readiness comes before motion.
Start conditions come before motion
When a machine will not start, check broad readiness conditions before diving into components: power, control power, safety ready, correct mode, utilities, faults reset, sequence at a valid starting state, external permissives, and product/material conditions. One missing readiness bit can block every output.
A packaging machine may be electrically healthy but remain stopped because air pressure is below minimum. Another may be in manual mode after maintenance and therefore ignore the automatic start command.
Field habit: Use the HMI status screen or logic to identify which readiness condition is false, then verify it physically.
Common trap: Do not replace a start button until you confirm the start command is actually missing.
Find whether the command enters the controller
Pressing a button creates a physical event that must reach an input and then be accepted by logic. If the input does not change, troubleshoot button, wiring, power, and input. If it changes but the sequence does not start, examine permissives and state logic.
This simple split prevents you from opening the power circuit when the machine never issued a run command.
Field habit: Watch the start input while the command is given, then follow the logic branch that should respond.
Common trap: Do not assume the HMI animation proves the PLC received the command.
Quick check
What should be true at this point in the sequence?
What evidence can prove or eliminate this section of the system?
What changed recently or only under the failing condition?
Look for a machine that is not in a startable state
Automatic sequences often require axes homed, cylinders retracted, products cleared, timers reset, or a previous cycle completed. After a jam or manual intervention, the physical machine can be between states that the program does not accept.
A pusher may be physically halfway out while both end sensors are off. The sequence cannot decide whether it is extended or retracted, so start remains blocked.
Field habit: Compare actual positions with the machine’s home or ready definition.
Common trap: Do not manually move components without understanding stored energy and sequence consequences.
Power path still matters
If the controller commands the first output but nothing moves, shift your attention to output hardware, control power, contactor or drive enable, field protection, actuator energy, and mechanical load. This is where the generic ‘won’t start’ problem becomes a specific local fault.
The first motor output may be on, but an open field fuse may leave the contactor coil dead. Or the contactor may pull in while the motor breaker is open.
Field habit: Walk the energy chain until you find where the command or energy stops.
Common trap: Do not keep staring at PLC logic after the correct output has already been proven.
Quick check
What should be true at this point in the sequence?
What evidence can prove or eliminate this section of the system?
What changed recently or only under the failing condition?
Field Exercise
Take one machine and make your own no-start readiness checklist.