How to Test a Proximity Sensor With a Multimeter (Step by Step)

When a machine stops because a sensor "isn't seeing" its target, the fastest fix is to find out in a few minutes whether the sensor, the wiring or the target is to blame. Here is the routine maintenance technicians use, for 3-wire PNP and NPN sensors and for 2-wire sensors.

What you need

• A digital multimeter set to DC volts.
• A metal target (for inductive sensors), such as a steel wrench.
• The sensor's part number or label, to confirm PNP, NPN or 2-wire. See PNP vs. NPN.
• Optional: a handheld tester such as HTM's S-TER-N sensor tester, which powers the sensor and shows its output on the bench.

Work safely: follow your site's procedures, and keep hands clear of moving machinery while the machine is powered.

Step 1: Watch the LED

Most sensors have an output LED. Move the target into the sensing zone, about half the rated range, and away again.

• LED toggles: the sensor is powered and detecting. The problem is more likely the output wiring or the PLC input.
• LED never lights: check power (Step 2).
• LED always on: something is in range (mounting bracket, swarf, a nearby metal part), the face is damaged, or the output is shorted.

Some sensors have a second, green power LED. If it is off, the sensor has no supply.

Step 2: Check the supply voltage

Measure between brown (+) and blue (0V) at the sensor or its connector (M12 pins 1 and 3). You should read the supply, typically 24 V DC. No voltage means a blown fuse, a cut cable or a bad connector; a low reading can mean a failing power supply or a voltage drop on a long cable.

Step 3: Check the output

Leave the sensor connected to its PLC input (the input acts as the load). An open output with no load can read misleadingly, so test with a load connected.

For a normally closed sensor the readings are reversed. If the LED changes but the output voltage does not, the sensor's output stage has failed; replace the sensor.

Step 4: Check the PLC input

If the sensor output switches correctly but the PLC does not see it, measure at the PLC input terminal with respect to the input common. Common causes are a PNP sensor on an input wired for NPN (or the reverse), a missing common, or a blown input. See sinking vs. sourcing inputs.

Testing a 2-wire sensor

A 2-wire sensor is in series with its load, like a switch. Measure across the load (the PLC input): with the target present, you should see the supply minus a few volts of sensor drop; with the target absent, close to 0 V. A small voltage when OFF is normal leakage. Never test an AC 2-wire sensor by connecting it directly across the supply without a load; it will be destroyed. See how to wire a proximity sensor.

Common reasons a sensor stops working

• Face damage: the target or a part struck the sensor face.
• Cable damage: crushed, cut or flexed beyond its rating. Consider a PUR cordset for moving cables.
• Target out of range: mount moved, or a different material. Aluminum and stainless are detected at much shorter distances than steel.
• Mounting: an unshielded sensor installed flush in metal. See flush vs. non-flush.
• Weld spatter or heat: use weld-field immune or PTFE-coated sensors near welding.
• Connector water ingress: loose or unsealed M12 connectors.

Replacing the sensor

Match the housing size, sensing range, shielded or unshielded, output (PNP/NPN, NO/NC) and connection. Send us the part number from the old sensor and we will find an exact or equivalent replacement. Browse HTM inductive sensors, call 1-800-724-4209, or request a quote.