Limit Switch Diagnostic

Test your furnace limit switches to ensure they are working normally.

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Limit switches are a critical safety feature of mid- and high-efficiency furnaces. A limit switch prevents the furnace from turning on or running if the internal furnace temperature exceeds a safe threshold, and automatically resets when the furnace cools down.

Refer to the furnace glossary if you need help identifying furnace parts or terms.

If your furnace shows a limit switch error, there are two likely scenarios:

  1. Furnace overheated — Overheating is a common problem, most often caused by reduced air flow.
  2. Limit switch failed — Less common, but can happen if the furnace has been short cycling due to reduced air flow for a long time. Repeatedly tripping and resetting eventually causes the limit switch to wear out and fail. Limit switches usually fail open, which will prevent the furnace from running at any temperature.

To determine the cause you should check for reduced air flow before testing the limit switch itself.

Never bypass a limit or roll out switch under any circumstance.

Bypassing a limit or roll out switch can cause your furnace to overheat and catch fire, causing injury and death.

Check for reduced air flow

Furnace overheating can be caused by a dirty furnace filter or blocking as little as 25% of the supply or return air vents. After you’ve done basic troubleshooting, check all of the following for signs of reduced air flow:
  • Furnace filter — replace if blocked or very dirty
  • Supply air vents — check for obstructions or blockages
  • Return air vents – check for obstructions or blockages

Interpreting the results

Determine which of the following cases matches your result.

Blockages found

If you found airflow obstructions or blockages, remove them and allow the furnace to cool down with the blower motor running for at least 10 minutes. Once the limit switch resets, the furnace should attempt to start up again. If the furnace runs without another limit switch error, then the limit switch is working properly and reduced air flow was the cause. If the furnace continues to show a limit switch error code, even after removing airflow obstructions or blockages and allowing it to cool down, proceed to Testing the limit switch below.

No blockages found

If you didn’t find any obstructions or blockages, but the furnace continues to show limit switch errors, the blower motor or its run capacitor may have failed. Go to the blower motor diagnostic to continue diagnosis.

Testing the limit switch

Use this procedure to diagnose if the limit switch itself is the source of your furnace problem — after you’ve done basic troubleshooting and checked for reduced airflow above.

Tools needed

  • standard multimeter that can measure continuity or resistance

Testing procedure

  1. If the blower motor is running, let the furnace continue to cool down for at least 10 minutes.
  2. Turn off the power to the furnace.
  3. Locate the primary limit switch. It is generally located in the upper furnace cabinet, below the burners, behind the gas valve and gas manifold. The switch will be a 1” x 1.5” rectangular piece of fibreboard, or round and about the size of a nickle.
  4. Locate the two wires that run to the limit switch, which have female friction-fit spade connectors. Disconnect both wires with a firm pull.
  5. Set your multimeter to measure continuity or resistance.
  6. Place one multimeter probe on each of the exposed male connectors on the limit switch.
  7. Record the resistance reading on the multimeter. If your multimeter set to test continuity, an audible sound indicates there is continuity across the limit switch.
  8. Reconnect the wires to the limit switch and reassemble the furnace.
  9. Turn on the power to the furnace.
Note: Some furnaces have multiple limit switches, wired in series with the primary limit switch. If multiple limit switches are present, you need to test each one using the procedure above. To locate the other switches, trace the wires leading to and from the primary limit switch. Additional limit switches may be located on the blower motor housing in the lower furnace compartment. There may also be roll out switches wired in series with the limit switch(es). When open, these roll out switches can cause the same error code. If present, use the roll out switch diagnostic to test them.

Interpreting the results

Determine which of the following cases matches your result.

Measured 0 ohms resistance or confirmed continuity

If you measured 0 ohms of resistance or confirmed continuity across the limit switch, then the limit switch is closed and working normally.

Infinite resistance or open circuit

If you measured infinite resistance or an open circuit, then the limit switch is faulty and needs to be replaced.

Next steps

If you have confirmed the limit switch is working normally, but continue to get the same open limit switch error code, check if there are other limit switches located in series with the primary limit switch and test them using the same procedure.

If, after confirming that all limit switches are working normally, you reassemble the furnace but continue to get open limit or roll out switch errors, continue to the roll out switch diagnostic.

If, after confirming that the roll out switches are also working normally, you continue to get open limit or roll out switch errors, the furnace control board may be faulty and need replacement. At this point, you should call a professional technician to diagnose the furnace.

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Furnace repairs should only be performed by a certified technician.

This website is for informational purposes only. Advanpro Ltd. is not liable for property damage, personal injury, or death caused directly or indirectly by actions taken as a result of the information provided here.

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