Won't Shut Off: A Cross-Trade Decision Tree

Why this matters

A system that runs nonstop burns energy, wears parts early, and sometimes creates a safety hazard (overheating, flooding, overcharging). Owners panic about the equipment when the real fault is usually a control that never told the system to stop. Knowing the order to check things keeps you from swapping an expensive component to fix a stuck switch.

Start here: is it really running, or just powered?

First confirm the unit is actually doing work, not just energized. A pump humming with no flow, a fan spinning with no heat, or a relay holding closed all look like "running" to a customer. Put a hand, a clamp meter, or a gauge on the output. If there is no real output but the unit is energized, you have a control or relay fault, not a load problem. If there is real output, the system thinks it still needs to run.

Branch 1: the system thinks it has not reached setpoint

Most "won't shut off" calls are a sensing problem. The system keeps running because its controller never sees the target condition met.

• Check the sensor or thermostat reading against reality. Compare the displayed value to an independent meter (thermometer, pressure gauge, scale). If the controller reads far from actual, the sensor is drifted, miswired, or in the wrong spot.
• Check sensor placement. A sensor in a drafty, sunlit, or dead-air location reads a false condition forever. The unit chases a target it can never satisfy at that spot.
• Check the setpoint itself. A target set beyond what the equipment can reach (too cold, too hot, too high a level) means it runs flat out and never stops. Confirm the setpoint is realistic.

If the reading is wrong, correct the sensor or its placement. If the reading is right and setpoint is realistic, move on.

Branch 2: a stuck switch or relay holds the circuit closed

If the control is satisfied but the unit still runs, suspect a contact welded or stuck closed.

• Listen for the call to stop. Force the setpoint so the controller should de-energize. If the controller drops its output signal but the load keeps running, the switching device downstream is stuck.
• Check the relay or contactor. Contacts pit and weld over time, especially on motor loads. A welded contactor passes power even with no coil signal. Verify coil voltage is absent while line voltage is still present on the load side.
• Check manual switches and bypasses. Someone may have jumpered a control or left a hand/auto switch in HAND. A bypass that was installed for a service call and never removed will run a unit forever.

Branch 3: a safety or limit that should stop it has failed

Many systems rely on a high-limit, float, or pressure switch as the normal shutoff. When that device fails closed, the primary stop is gone.

• Test the limit device. Simulate the trip condition and watch whether the switch opens. A limit that never opens leaves the system with no upper boundary.
• Check for a defeated safety. Limits get bypassed during troubleshooting and forgotten. Restore every safety to service before you leave.

Branch 4: short cycling that looks continuous

Sometimes the unit is cycling so fast it appears to run constantly. Rapid restarts usually come from a sensor with no deadband, a hunting control, or an oversized system. Watch the load over a few minutes. True continuous run points at Branches 1 to 3. Rapid on/off points at control tuning or sizing.

What to fix first, in order

1. Confirm real output versus just powered.
2. Verify the sensing element and its placement.
3. Confirm the setpoint is achievable.
4. Test the switching relay or contactor for a stuck-closed contact.
5. Test the safety/limit that should normally stop it.
6. Remove any leftover jumper or manual bypass.

Work cheapest and most common first. The welded contactor and the drifted sensor cause far more of these calls than a failed main control board.

References

• NFPA 70 (National Electrical Code), control circuit and disconnect requirements
• OSHA lockout/tagout (29 CFR 1910.147) for safely de-energizing before contact testing
• Manufacturer control and sequence-of-operation documentation
• See related: Universal "How Motors Fail: The Common Modes"