Onsite Siemens controls diagnostics

Siemens PLC in RUN but the machine won't start?

The controller may be executing its program while automatic operation remains unavailable. That is a different problem from a PLC that has entered STOP. West Engineering Systems investigates the available machine, PLC, HMI, I/O and drive evidence onsite to identify where operation is being held up and help determine the next action. We are based in Appleton, Wisconsin, and provide field service across the United States.

Discuss Your Machine-Start Problem

Prefer to describe the fault by phone? Call (920) 851-6363.

RUN is controller status, not a machine-ready verdict

Siemens describes RUN as the mode in which an S7-1200 executes its user program. That does not establish that an operator start request was accepted, that the required conditions for a particular sequence are met, or that an output and its field device have responded. Those questions depend on the machine's actual design. See Siemens' S7-1200 program-execution documentation.

PLC shows RUN; start does nothing

Record what the operator requested, which mode was selected and whether the HMI displayed a message. A start request, an operating-mode selection, or a machine-specific permissive may be missing; the CPU's RUN indication alone cannot distinguish them.

A sequence begins, then stops

Note the last completed action and what was expected next. An output command, device response, sensor feedback or sequence condition may need investigation. Do not assume the component nearest the stop point is faulty.

HMI message or drive fault appears

Preserve the exact alarm text and order of events. A message may name a blocking condition, a communication problem or a downstream symptom; it is evidence to compare with the machine state, not a complete diagnosis.

Different symptom? If the CPU actually reports STOP, use the S7-1200 CPU STOP / PROFIBUS case for an example of diagnostic-buffer evidence in a confirmed machine all-stop. That case does not identify the cause of a machine whose PLC remains in RUN.

What an onsite investigation can compare

The objective is to locate the first condition that differs from the intended sequence, not to guess a replacement part from a single alarm. Depending on the installed equipment, documentation and authorized access, the investigation may compare:

  • Operator request and machine state: Is the requested operating mode available? What happens on the HMI, and at what step does progress stop?
  • PLC and I/O evidence: What status, diagnostic buffer entries and device indications are available near the time of the event? Siemens documents the S7-1200 diagnostic buffer as a record of recent CPU events; an empty or unrelated entry does not prove that the machine sequence is healthy.
  • Sequence conditions: An authorized controls professional can trace the start request through relevant machine-specific permissives, interlocks, commands and feedback in the actual project. Siemens' TIA Portal watch-table documentation describes online monitoring of tag values; those values need to be interpreted against the installed program and machine state.
  • Drive or field-device response: If a command is expected, distinguish a missing command from a displayed drive fault, unavailable device or missing feedback before concluding where the fault lies.

These are possible diagnostic areas, not a claim that any particular permissive, sensor, drive or network has failed. Online monitoring and configuration access depend on site authorization and the available engineering project. This page is not a procedure for changing safety logic or restarting a stopped machine.

What to gather before calling

Share what operators and maintenance staff can already observe, if available:

  • Machine location, machine name, affected station or axis, and whether the problem is constant or intermittent.
  • The observed PLC RUN/STOP indication, what operating mode was selected, the operator action requested, and the last machine action that completed.
  • Exact HMI messages and drive fault or warning codes, with photos of existing displays and the approximate order and time they appeared.
  • Whether any part still operates as previously observed; report existing observations rather than operating equipment just to reproduce the fault.
  • Recent changes, power events or prior troubleshooting attempts, plus available drawings, machine documentation and an unmodified project backup if the site has one.
  • A site contact who can authorize access and provide the PLC, HMI and drive models if already known. Do not open a panel merely to obtain a model number.

Related Siemens troubleshooting paths

If the CPU status or initiating fault is still unknown, start with the Siemens machine-down diagnostic guide, which covers PLC, HMI and drive evidence together. If the issue centers on an axis that jumps or faults as it is commanded, the SINAMICS S120 positioning-axis case documents a different, case-specific drive investigation.

If the machine requests motion but a SINAMICS G120 drive will not start or displays a fault, see the G120 drive-start guide for drive-side evidence to gather; first establish whether the drive actually received a request.

Scope of service: An onsite visit is for troubleshooting, diagnostics and fault identification to help determine the next action. Findings depend on access and the evidence available; a restart, part replacement or equipment repair is not promised.

Need onsite Siemens PLC troubleshooting?

Tell us where the machine is, what it will not do, and what the PLC and operator display show. West Engineering Systems can discuss whether an onsite diagnostic visit is a fit and help identify the next action.

Request Service

Or call (920) 851-6363 or email info@westengineeringsystems.com.