On the covered Rheem and Ruud standard gas furnace, 3 blinks means limit switch open: the limit control circuit is open. Rheem's FAQ says the main high limit may be open due to low air flow or an over-fired heat exchanger, or the manual-reset rollout switch above the burner may have tripped. First, check that the air filter is clean and the supply and return registers are open.
Differential
Likely causes of “Rheem and Ruud standard gas furnace showing 3 blinks”, and how to tell them apart
- 1
The limit switch circuit is open (three blinks)
What separates it The furnace starts and then shuts down, or does not start, with three blinks
How to confirm Check that the air filter is clean and the registers are open
- 2
Low air flow across the heat exchanger
What separates it A dirty filter or closed registers restrict the flow
How to confirm Replace the filter and open the registers
- 3
Excessive manifold gas pressure (an over-fired heat exchanger)
What separates it The furnace overheats when it runs
How to confirm A technician checks the gas pressure
- 4
The manual-reset rollout switch above the burners has tripped
What separates it The switch has a reset button
How to confirm A technician finds why it tripped
- 5
A blocked or damaged heat exchanger or vent
What separates it Combustion air is limited
How to confirm A technician inspects the furnace
Before you touch a setting
Record this first — changing settings destroys the evidence
- That the LED shows three blinks
- When the furnace shuts down
- The state of the filter
- The model number
Try in this order
0 of 9 completedWhat to do first
ReversibleCheck that the air filter is clean and the supply and return registers are open.
Correct only the documented fault domain
ReversibleRheem's FAQ: an open high limit can come from low air flow across the heat exchanger or excessive manifold gas pressure, and a rollout switch can trip from a lack of combustion air, a blocked heat exchanger or vent, or a damaged heat exchanger. It says to check for excessive manifold pressure and a blocked flue or heat exchanger. A tripped rollout switch is a safety condition, so leave it to a technician.
Check the air filter
ReversibleReplace a dirty filter, since low air flow is the commonest cause.
Open all supply and return registers
ReversibleMake sure that none is closed or blocked by furniture.
Check the blower
ReversibleListen for the blower to run when the furnace calls for heat.
Look for a rollout switch
ReversibleA tripped manual-reset rollout switch above the burners points to a combustion air or heat exchanger problem.
Do not reset repeatedly
ReversibleResetting a rollout switch without finding the cause is unsafe.
Call a technician
ReversibleRheem says to check for excessive manifold pressure and a damaged heat exchanger, which are technician tasks.
Plan a yearly service
ReversibleA heating technician can check the heat exchanger, the gas pressure and the venting once a year, which prevents many limit faults and catches a cracked heat exchanger early.
Model-specific
What is true of this model and not of its siblings
- The code on this page
- Three blinks, limit switch open.
- Parts involved
- The limit switch, the rollout switch, the filter and the heat exchanger.
- Rheem's guidance
- Check the filter and registers, then call a technician.
- What the limit switch protects
- The high limit switch senses the temperature of the heat exchanger and opens if it gets too hot. That shuts off the gas so that the heat exchanger is not damaged. It is a protective device that does exactly what it is meant to do, and the cause of the overheating is the thing to fix.
- The cheapest fix first
- A dirty filter is by far the commonest cause of an open limit. Changing the filter takes two minutes and costs very little, so do it before you call anyone, and set a reminder to change it every one to three months.
Stop boundary
When this stops being a self-service repair
- !Rheem's FAQ says an open high limit can come from low air flow or excessive manifold gas pressure, and a rollout switch can trip from a lack of combustion air, a blocked heat exchanger or vent or a damaged heat exchanger
- !Do not reset a rollout switch repeatedly
- !If you smell gas or exhaust, leave the house and call the gas company
Evidence ledger
Sources behind this answer
Exact questions
Common questions
What does 3 blinks mean on a Rheem and Ruud standard gas furnace?+
Limit switch open: the limit control circuit is open. Rheem's FAQ says the main high limit may be open due to low air flow or an over-fired heat exchanger, or the manual-reset rollout switch above the burner may have tripped.
What does three blinks mean on a Rheem standard gas furnace?+
The limit switch is open.
What causes an open limit?+
Low air flow, excessive gas pressure or a rollout switch that tripped.
Is it safe to run the furnace?+
Not repeatedly, since overheating can damage the heat exchanger.
What is the first check?+
The air filter and the registers.
How often should I change the furnace filter?+
Every one to three months, depending on the filter and on pets, dust and use.
Can closed registers cause three blinks?+
Yes, closing many registers reduces the flow across the heat exchanger and can make the limit open.