Why Does a PLC Input Stay On After the Sensor Turns Off?

Key Specifications

Types & Variations

Maintenance Steps

1. Make the machine safe and capture the symptom
   Follow the site energy-control procedure. Apply lockout/tagout (LOTO), isolate electrical and stored mechanical energy, bleed down pneumatic and hydraulic pressure, block gravity loads, and verify zero-energy state. Record the sensor LED, module LED, raw input tag, channel, terminal, and assigned common before moving a single wire.

   A PLC stop command is not energy isolation. Stored pressure, elevated tooling, charged capacitors, and remote starts can all still injure personnel.

2. Measure input-to-common voltage and current
   Energized diagnostics belong to a qualified person working under the facility's approved electrical-safe-work procedure. Use a properly rated meter and probes, take the reading at the input terminal against its own assigned common with the sensor on and off, and compare both numbers with the exact module data sheet limits.

   Do not move conductors or insert an ammeter in series on a live control circuit. De-energize first — an open meter lead can create an unexpected machine command.

3. Lift the signal wire and divide the circuit
   Reapply LOTO, prove de-energized, mark the conductor, and remove it from the input terminal. Secure the loose end. Under controlled power, observe whether the empty channel releases, then repeat the split at the nearest field junction box when the fault follows the cable.

   Restore guards, clear personnel, and control every restart. Never leave a loose conductor where it can touch supply voltage or another input.

4. Fit and prove the correction
   Repair the identified path, or install the calculated bleeder or interface relay. Check terminal seating and torque against the component instructions, cycle the sensor repeatedly, watch the raw input status, and retest at the low and high ends of the normal supply range and at operating temperature where practical.

   A bleeder dissipates heat continuously in one sensor state. Do not bury it in a wire duct, and do not choose wattage from resistance alone.

Read the terminal against the module's two published limits

Put the meter from the suspect input terminal to its own assigned input common, never to the cabinet ground bar, and record the voltage with the target absent. Then compare that number against the module data sheet, which publishes two limits rather than one. An OMRON CP1L normal input is guaranteed OFF at 1 mA max at 5.0 VDC max, guaranteed ON at 14.4 VDC min, and has a 4.7 kΩ input impedance. Everything between 5.0 V and 14.4 V is undefined. That undefined band is where most stuck inputs actually sit. Voltage alone still does not close the case. A high-impedance meter reads coupled voltage on an open conductor carrying almost no current, and that ghost collapses the moment a real load is applied. Current is the honest measurement: residual voltage backed by more than the module's OFF current rating can hold the optocoupler, and a ghost cannot.

Check the input type before calling it a defect

EN 61131-2 defines three types of current-sinking digital input. Beckhoff's application note on the standard says of Type 1 that it "is therefore only suitable to a very limited extent or not at all for the use of 2-wire sensors, for which a high quiescent current is required." That one sentence resolves a large share of these calls. A two-wire proximity or photoelectric sensor draws current while off simply to power its own electronics, so the residual current is designed in rather than symptomatic. Type 2 was written for those sensors. Type 3 came later and carries the same quiescent current at lower power dissipation. Land a two-wire device on a Type 1 channel and nothing is broken — the pair was never compatible. The threshold gap looks small in print and decides the case in the panel. Type 1 does not treat the line as high until 15 V, while Types 2 and 3 do so from 11 V. Read the card's type from its own specification, not from the fact that it accepted 24 V.

Size the bleeder with the manufacturer's two equations

A bleeder resistor across the input diverts leakage so the terminal falls back into the guaranteed OFF window. OMRON publishes the sizing directly in the CP1L two-wire sensor precautions, and the trigger condition is a comparison, not a guess: connect one only when the sensor leakage current Ileak exceeds the module OFF current IOFF. Resistance comes from R ≤ RIN × VOFF/(Ileak × RIN − VOFF), and dissipation from Power W ≥ (VCC − VR)² / R × 4, where the 4 is OMRON's stated allowable margin. Work it with the CP1L numbers above and a sensor specified at 1.7 mA leakage: R ≤ 4.7 kΩ × 5.0 V / (1.7 mA × 4.7 kΩ − 5.0 V) gives 7.9 kΩ, so 6.8 kΩ is the next standard value down. Wattage then runs (24 V − 3 V)² / 6800 Ω × 4, about 0.26 W, which makes a 0.5 W part the floor and a 1 W part the sensible pick in a warm enclosure. Note where the heat goes: the resistor dissipates continuously in one sensor state, so it belongs on a terminal or bracket with air around it, never buried in a wire duct.

The cases a bleeder will not touch

Resistance across the input is the right answer only for leakage that is supposed to be there. A wet or condensing junction box, crushed insulation, or conductive dust creates a path that grows as conditions worsen, and adding a bleeder hides it until it grows past the bleeder. Long signal conductors run in the same tray as contactor coils, solenoids, or VFD motor leads couple voltage capacitively into an open wire — that one is the classic ghost, and it usually disappears under load rather than needing a resistor. Then there is the group that has nothing to do with the field at all: a forced tag, a latched seal-in rung, a stale HMI screen, or a channel mapped to the wrong address. Lift the field conductor at the terminal under lockout, restore power under the site's controlled procedure, and watch the empty channel; an indication that stays on with nothing landed has already ruled the sensor and cable out. One firm exception overrides all of the above — never place a bleeder, jumper, or any diverting component on a safety input or guard circuit. Those faults go back to the safety system manufacturer's approved diagnostic method. OMRON's manual flags one more first-day trap worth knowing: sensor inrush at power-up can register as a false input before the sensor stabilises, and a short program timer after sensor power is the intended fix.

FAQ

References

• OMRON, CP1L CPU Unit Operation Manual (W471)
• Beckhoff, Application Note DK9222-0909-0008 — Digital inputs for 2-wire and 3-wire sensors according to EN 61131-2
• IEC, IEC 61131-2 Programmable controllers — Part 2: Equipment requirements and tests
• OSHA, The control of hazardous energy (lockout/tagout), 29 CFR 1910.147