HOW-TO GUIDE
How to Check a Thermocouple With a Multimeter
By the OhmSavvy Research Team ·
Gas appliances and hot parts
- Turn the gas control to off and let the burner area cool before removing or disconnecting a pilot thermocouple. If you smell gas, stop and leave the area.
- Never bypass, bridge or hold in a gas safety valve to keep a burner lit. If you are not sure about working on a gas appliance, call a qualified technician.
- Heat a loose test probe with boiling water, a hair dryer or a lighter held briefly at the tip, not for long periods. Keep the meter and its leads away from the flame.
Thermocouple vs thermistor: what you are actually testing
The two sensors are often confused, but a meter treats them very differently. A thermistor is a resistor whose value changes with temperature, so its resistance reading is the measurement. A thermocouple is a pair of dissimilar metal wires that generates its own small voltage, and its resistance only tells you whether the circuit is complete. If you have a two-wire sensor reading kilohms, it is almost certainly a thermistor; follow our guide to testing a thermistor with a multimeter instead.
| Thermocouple | Thermistor | |
|---|---|---|
| Output | Self-generated millivolts | Resistance, needs a test current |
| Typical resistance | A few ohms or less (longer fine wire reads more) | Hundreds of ohms to hundreds of kilohms |
| Meter function to check it | Ω for continuity, mV DC for output | Ω only |
How a thermocouple makes a voltage
Where two different metals are joined, a temperature difference along the wires produces a small voltage, known as the Seebeck effect. The reading depends on the gap between the hot junction at the tip and the reference junction where the wires meet the meter terminals, not on the tip temperature alone.
1Test 1: check thermocouple continuity and resistance
Disconnect the thermocouple from its controller or gas valve, then set the meter to its lowest resistance range. Touch the probes together and note the lead resistance, then measure across the two thermocouple leads.
- Low reading: typically a few ohms for a short probe, more for long or very fine wire. A pilot thermocouple usually reads well under an ohm. This means the junction is joined.
- OL: the circuit is open, usually a broken tip weld or a wire snapped inside the sheath. Replace it.
- Sheath check: on an ungrounded probe, each lead to the metal sheath should read OL. Continuity there means the junction is touching the sheath, which can upset readings on some controllers.
Flex the cable along its length while watching the display. A reading that flickers to OL points to a break in the wire; the method in testing electrical wires applies equally to thermocouple extension cable.
2Test 2: the millivolt test when heated
Switch to DC millivolts (the mV position, or the lowest DC volts range on a manual meter). Connect the red probe to the positive lead and the black to the negative. At room temperature the reading should sit near 0 mV. Now heat the tip and watch it climb steadily; remove the heat and it should fall back. A reversed reading only means the leads are swapped.
A worked example: dip a Type K tip into boiling water at about 100 °C while the meter terminals sit at 22 °C. The standard Type K table gives 4.096 mV at 100 °C and 0.879 mV at 22 °C, so expect about 3.2 mV. That matches the quick estimate of 78 °C × 41 µV/°C.
Meters with a temperature function
Type K thermocouple millivolt values
These are standard Type K reference values with the cold junction at 0 °C. With the meter at room temperature, subtract roughly 1 mV (the value at 25 °C) to estimate what a plain mV reading should show.
| Tip temperature | Type K output (0 °C reference) |
|---|---|
| 0 °C | 0.000 mV |
| 25 °C | 1.000 mV |
| 50 °C | 2.023 mV |
| 100 °C | 4.096 mV |
| 200 °C | 8.138 mV |
| 300 °C | 12.209 mV |
| 500 °C | 20.644 mV |
How to check a furnace or water-heater thermocouple
A standing-pilot furnace, boiler or water heater uses a thermocouple in the pilot flame to hold its gas valve open. If it weakens, the pilot lights while you hold the button but goes out when you let go. To test it, unscrew the thermocouple nut from the gas valve, relight the pilot following the appliance instructions, and measure between the tip of the connector and the copper tube with the meter on mV DC.
If a boiler fires normally but the heating never comes on, the room thermostat is the next suspect: see how to test a boiler thermostat with a multimeter.
- Open-circuit: typically 20 to 30 mV once the flame has heated the tip for a minute or so. Well below about 20 mV usually indicates a worn thermocouple or a weak, poorly aimed pilot flame.
- Under load: with an adapter fitted between thermocouple and valve, readings are typically about half the open-circuit figure. Use the control manufacturer's limits.
- Thermopiles on millivolt gas fireplaces stack many junctions and typically produce several hundred millivolts instead.
Modern furnaces with a hot-surface igniter usually have a flame-rectification rod rather than a thermocouple, and that is checked as microamps DC in series, using the meter's µA range. Our guide to measuring milliamps with a multimeter covers the same series hookup.
Reading the result
| Result | Meaning |
|---|---|
| Low ohms, mV rises smoothly with heat | Good thermocouple |
| OL on resistance | Open junction or broken wire |
| Low ohms, but little or no mV when heated | Degraded tip, or heat not reaching the junction |
| Pilot type under 20 mV open-circuit | Weak thermocouple or a poor pilot flame |
Which multimeter you need for thermocouple checks
You need a DC range that resolves tenths of a millivolt, which most digital meters offer. Extra display counts help when a few degrees is under 0.2 mV. For reading accuracy specs, see multimeter accuracy for electronics repair, and for a refresher on the dial and jacks, see how to use a multimeter.
If most of your thermocouple work is on furnaces, a meter with both a Type K input and a DC microamp range covers the pilot thermocouple and the flame-rectification rod with one tool. For bench probes and oven sensors, a high-resolution mV mode matters more than HVAC extras.
Meters for thermocouple and pilot checks
Hands-on tested. A 19,999-count high-resolution mode shows small mV changes in finer steps.
Type K input (-40 to 400 °C) plus a 0.1 µA range for flame-rod checks. No mA or amp ranges.
Budget True RMS meter that reads temperature to 1832 °F with the thermocouple in the box.
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Frequently asked questions
How many millivolts should a thermocouple put out?
It depends on the type and the temperature difference. A Type K produces roughly 41 µV per °C, so about 4 mV at 100 °C above a 0 °C reference. A gas-appliance pilot thermocouple typically gives 20 to 30 mV open-circuit with the pilot lit. Check the appliance or control manufacturer's figure for a pass or fail limit.
Can a thermocouple test fine on resistance and still be bad?
Yes. Continuity only proves the circuit is closed. A tip that has oxidized or been damaged by heat can still read a few ohms but produce too little voltage, or a voltage that wanders. That is why the millivolt test with heat applied is the one that decides whether a thermocouple is still usable.
Why does my thermocouple read 0 mV at room temperature?
Because a thermocouple only measures the difference between its tip and the point where it connects to the meter. When both are at room temperature, there is no difference and the output is close to zero. Heat the tip and the reading should start climbing within seconds. A plain multimeter does not add cold-junction compensation on its mV range.
Which wire is positive on a Type K thermocouple?
In the North American color code the yellow lead is positive and the red lead is negative; red is the negative lead on every ANSI thermocouple type. Other regions use different color codes. On bare Type K wire, the negative alumel leg is slightly magnetic, which helps identify it if the insulation colors are unclear.
Is a furnace flame sensor the same as a thermocouple?
Not on most modern furnaces. A pilot thermocouple generates millivolts from heat, while the single-rod flame sensor on many hot-surface-ignition furnaces works by flame rectification and is checked as a small DC current in microamps, with the system running. Look at the part and its wiring before choosing the test, and follow the furnace manual.
