Rheem Furnace 4 Flashes - Causes & Fix

Rheem Furnace 4 Flashes — What It Means

A 4-flash code on older LED-style Rheem and WeatherKing furnaces indicates the high-limit circuit is open. This means your furnace overheated and the limit switch tripped to protect the heat exchanger and cabinet from damage. The control has stopped burner operation until the problem is corrected.

The limit switch is a normally closed safety device that opens when cabinet temperature rises above safe levels. When it opens, the circuit is broken and the furnace cannot fire. The overwhelming majority of 4-flash limit trips are caused by restricted airflow that keeps heat from being carried away from the heat exchanger.

Jump to Fix

Common Causes

• Dirty or clogged air filter The most common cause by far is a filter that restricts return airflow and traps heat in the cabinet.
• Blocked supply or return air Closed registers, furniture blocking return grilles, collapsed ductwork, or shut dampers can all choke off airflow and cause overheating.
• Blower not moving enough air A failing blower motor, weak run capacitor, dirty blower wheel, or incorrect speed tap can reduce airflow below what the furnace needs to stay within limit.
• Failed or intermittent limit switch If airflow is normal but the circuit still opens, the limit switch itself may be failed and stuck open or opening prematurely.
• Blower control or wiring fault Loose connections, damaged wires, or control board faults can prevent the blower from starting or running at proper speed during the heat cycle.
• Flue or vent blockage Obstructions in the vent or combustion-side issues can contribute to overheating, though this is less common than airflow restrictions.

Step-by-Step Fix

1. Verify the code by counting the LED flashes on your furnace control board and matching them to the diagnostic chart on the unit or in your model’s literature to confirm 4 flashes indicates open high limit.
2. Inspect and replace the air filter if it is dirty, clogged, or collapsed, then check that all return-air grilles and supply registers throughout the house are open and unobstructed.
3. Check blower operation by turning on the furnace and listening for the indoor blower to start and run at full speed during the heat cycle, and visually inspect the blower wheel for dust buildup or damage.
4. Test the blower capacitor with a multimeter if the blower is running slowly or struggling, and replace it if readings are out of range or if the capacitor shows bulging or leaking.
5. Inspect wiring and connections at the limit switch and control board for loose, burnt, or heat-damaged terminals, and reseat or repair any faults you find.
6. Test the limit switch electrically by checking continuity across its terminals at room temperature (it should be closed), and replace the switch if it is open when the furnace is cool or if airflow is confirmed normal but the limit still trips.
7. Clear the fault and test by restoring power, running the furnace through a complete heat cycle, and monitoring cabinet temperature and blower performance to confirm the limit no longer opens.

Parts Often Needed

When to Call a Pro

Call a qualified HVAC technician if the 4-flash code returns after you have replaced the filter and confirmed all registers are open, if the blower does not start or runs weakly, or if you are not comfortable working with electrical components and gas-fired equipment. Also call for help if you find evidence of repeated overheating such as discolored metal, burnt wiring, or heat stress around the heat exchanger. A technician has the tools to measure temperature rise, check combustion, test the limit switch accurately, and diagnose blower or control board faults that are beyond typical homeowner troubleshooting.

See Also

• Rheem Furnace Error Code 57 — Causes & Fix
• Rheem RA13 Air Conditioner Error Codes — Fault Code Diagnostic Guide
• Rheem Furnace 4 Flashes — Open High Temperature Limit Fix
• Rheem Furnace 7 Flashes Error Code — Causes & Fix