Troubleshooting guide

Why Does a PLC Output Not Turn On the Field Device?

Logic, forced values, output module faults, blown output fuses, missing field power and wiring: a systematic way to find where the signal stops.

Controls & Instrumentation · Last reviewed

The path from program logic to a working field device has several links: the logic that decides the output, the output module, the field power, the wiring and the device itself. Work through the links in order and find the first one where the signal is missing.

Isolate and verify the absence of voltage before touching terminals. Take care before forcing outputs or making program changes on a running machine: a forced output can start motion or release stored energy.

What you seeWhere the problem isWhat to check
Output bit is off in the programLogicWhy the rung is false: inputs, interlocks, permissives, timers, and the mode (auto, manual)
Output bit is on, output LED is offOutput module or its configurationModule status LEDs, a blown output fuse, a module fault in diagnostics, or an output disabled by a fault
Output LED is on, no voltage at the terminalMissing field power or a failed output pointSupply to the output common, fuse, and the point itself (meter across the terminals)
Voltage is present at the terminal, device is offWiring or the deviceWiring continuity, terminal tightness, a failed relay or contactor coil, or a tripped overload or safety circuit
Device is on when it should be offForced value or a welded relay contactForce table, and a leaking output (see below)
Works intermittentlyLoose connection, noise, or a marginal power supplyTerminal torque, supply voltage under load, and shielding

A leakage example

Triac outputs and solid-state output points leak a small current (often 1 to 2 mA) when off. A small lamp or an electronic relay input with a high impedance can see enough voltage from this leakage to stay partly on. A bleeder resistor across the load, or a relay output, removes it.

A good order of attack

  1. Check the processor mode and any faults (run mode, no major fault).
  2. Monitor the logic online and see why the output bit is or is not set.
  3. Check the output module LED against the bit, then the module diagnostics.
  4. Measure at the terminal with a meter. No voltage means the problem is on the PLC side. Voltage means look at the wiring and the device.
  5. Check for forced values and clear them only with authorization.

A second example: sizing a bleeder resistor

A solid-state output leaks 2 mA when it is off, and a small load connected to it stays dimly on. The load needs to see less than 12 V (10% of 120 V) when the output is off.

  1. The resistor in parallel with the load must carry the leakage with a drop below 12 V: R ≤ 12 V ÷ 0.002 A = 6,000 ohm.
  2. When the output is on, the resistor sees the full 120 V and dissipates 120² ÷ 6,000 = 2.4 W.
  3. Choose a resistor of 5 W or more, for example 5 kilohm, so it runs cool.

Preventive checks

  • Document forced values and clear them as soon as the test is over.
  • Check output module fuses and the field power supply voltage on a schedule.
  • Keep a backup of the program and compare it with the running program after any change.
  • Label every output at the terminal and in the program so that the path can be traced fast.

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Questions

What is a force?

A forced value overrides the program and holds an input or output at a set state. It is a troubleshooting tool, and forces left active are a common cause of unexpected behavior.

Should I just replace the output module?

Not first. Confirm that the module is the failed link, because the cause is often the fuse, field power or wiring.

What is a sourcing and a sinking output?

A sourcing output supplies the positive voltage to the load. A sinking output connects the load to the common. The load must match the type.

Why does the LED show on but the device stay off?

The module LED often shows the logic state before the output stage. A failed output stage, a blown fuse or missing field power can leave the terminal dead while the LED is on.

Should I replace the output module right away?

Only after checking field power, the fuse and the wiring. Many output failures turn out to be a short in the field wiring that blew the fuse.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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