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The PMC, which is the PLC built into the control and the thing that governs all the machine logic, has thrown a system fault or its internal watchdog has stopped it. The machine does not come ready and it is not resolved from the panel. This is one of those alarms where the right move is to stop, touch nothing and call.
For reference only: alarms change with the manufacturer, the control and the machine PLC. Check the full code in the official manual before doing anything. Legal notice.
Always: requires control/PMC diagnostics.
Reset only if the manual says so; if it returns, stop.
Choose the manual matching your control series, software version and machine builder. A support portal is a starting point; it does not establish applicability to every model.
The PMC cyclically runs the machine logic program: it reads the inputs from sensors and pushbuttons, solves the ladder the builder wrote, and updates the outputs that command contactors, solenoid valves and sequences. That cycle has to complete within a set time, and an internal watchdog checks it: if a cycle does not finish on time, or if the processor detects a condition that should not occur, the system halts rather than carry on operating with logic whose state is unknown. The reason is obvious as soon as you think about what the PMC controls: tool changer, clamping, doors, pumps, gear ranges, machine safety. Logic running half-executed with that scope is exactly what nobody wants. The causes split between internal and external. Internal: corrupted PMC memory, damaged ladder, an I/O module that does not respond and leaves the cycle waiting, a failure of the processor itself. External: a badly powered I/O card, interrupted communication with remote modules, or a short circuit on an output that drags the module down. And one factor makes everything worse: the ladder is the machine builder's property and responsibility; modifying it without their documentation is not fixing anything, it is taking on responsibility for the machine's safety.
In order, from the fastest and cheapest check to the most expensive.
01Note the exact number and any additional screen
The specific number and, if the control offers a PMC diagnostic screen with more detail, whatever appears there. It is the information the service engineer needs and what stops them starting from scratch when they arrive.
02Collect the context
What the machine was doing, whether it was at start-up or while running, whether there was a power cut, whether it was in the middle of a tool change or a clamping sequence, and whether anything has been installed or replaced recently. A PMC fault that appears after an intervention points at that intervention.
03Do one single power cycle, by the procedure
Power down by the builder's procedure, wait and power up. If it was a transient, the machine starts normally. If it comes back, do not insist: switching off and on repeatedly with a hard PMC fault adds no information and can leave actuators in intermediate states.
04Check whether anything is mechanically half-done
If the alarm arrived in the middle of a tool change or a clamping sequence, an arm, a chuck or a clamp may be in an intermediate position. Do not try to move it by hand or force the sequence: that recovery has its own procedure in the machine manual and often requires service mode.
05Do not modify the ladder and do not force signals
The PMC signal forcing functions exist for service engineer diagnosis, not for getting by. Forcing an output can move an actuator with people nearby or skip a safety condition. And editing the builder's ladder without their documentation leaves the machine in a state nobody will be able to audit afterwards.
06Call maintenance or the service engineer with the data collected
Alarm number, context, recent interventions and the state the machine was left in. If a PMC and parameter backup exists, say so: it is the first thing they will ask and it determines whether the repair takes hours or days.
Accessory wiring or configuration affecting an I/O module, or a shorted output. It is confirmed by the timing. Whoever did the installation resolves it by reviewing their work, not the control manufacturer.
A sensor or module that does not respond and leaves the logic waiting, or a condition that is met only marginally. It is confirmed by the repeatability. Maintenance diagnoses it with the builder's documentation.
Memory, control unit supply or hardware degradation. It is confirmed by the absence of a pattern and by coinciding with other odd symptoms. It needs service engineer diagnosis and, almost always, a backup to restore from.
A PMC and machine parameter backup saved, dated and kept off the machine, made by the builder or by maintenance; without it, any fault of this kind is a long stoppage. A stable, protected supply, because power cuts in mid-cycle are a common origin. Documentation for any accessory installed, with its drawing and its signals. Thermal control and cleaning of the cabinet. And a clear shop rule: the ladder is not touched and signals are not forced.
Fanuc reports an internal fault or PMC/PLC watchdog. It is an alarm from the PLC/machine area, and in the catalog it is listed as “PMC system alarm / watchdog”. On the machine you notice it like this: Machine is not ready, PMC alarms at start-up, May block OEM sequences.
On the shop floor it almost always comes from one of these causes: PMC error, Watchdog triggered by the PLC scan, Corrupt memory/backup, Defective module or ladder. Rule out the most likely one on your machine first, before touching parameters or swapping anything.
Reset only if the manual says so; if it returns, stop. Resetting without fixing the cause first only hides the problem: Fanuc 950/951 trips again and, in the meantime, you keep working with the machine in bad shape.
With the machine stopped and in a safe state, start with these checks: Note number and PMC screen, Do not modify the ladder logic, Reset only according to OEM procedure. That alone will tell you whether it is the program or whether you need to raise a maintenance job.
With Fanuc 950/951 active, avoid this: Do not bypass safety devices, Do not manipulate energized electrical cabinet, Do not continue producing if the alarm reappears. Skipping any of these points is what turns a minor stop into an expensive breakdown.
Always: requires control/PMC diagnostics. If you are not sure how far it goes, stop the machine and report it: a repeat alarm always works out cheaper than a crash.
It is critical: there is a real risk of damage to the machine, the tool or a person. Leave the machine stopped, do not keep resetting it to see if it goes away, and call maintenance before producing again. Guidance reference. Always confirm the full text in the exact machine/control manual.
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