Rheem Furnace Won’t Turn On — What’s Happening
A Rheem furnace that won’t turn on is a symptom, not a single fault code. The control board will display a specific diagnostic code through LED blink patterns or a numeric display to tell you exactly why it refused to start. Common codes include ignition failure (11), flame sense issues (12 or 13), limit switch trips (22), pressure switch open (45 or 46), rollout faults (33), blower problems (61), or control board failures (93). Each code points to a different underlying problem, so you need to read the blink count or display first.
The furnace shuts down and won’t restart because it detected an unsafe or incomplete condition during the last cycle. It stays locked out until you fix the root cause and reset power or the thermostat. The most frequent culprits are dirty flame sensors causing false flame-loss readings, clogged air filters or restricted airflow tripping the high-limit switch, blocked vents or weak draft causing pressure-switch faults, and gas supply or ignition component problems preventing burner light-off.
Most Likely Causes
- Dirty flame sensor Carbon buildup or oxidation on the flame sensor rod prevents reliable flame proving, causing codes 12 or 13 and lockout after ignition or mid-cycle.
- Restricted airflow from dirty filter or blocked return Low airflow causes the furnace to overheat and trip the high-limit switch (code 22), shutting down the unit to prevent damage.
- Blocked intake or vent piping, weak inducer, or pressure-switch tubing obstruction Inadequate draft or combustion-air flow prevents the pressure switch from closing, resulting in codes 45 or 46 and no ignition sequence.
- Gas supply problems Closed manual gas valve, low inlet pressure, or failed gas valve prevents burners from lighting, causing code 11 ignition failure or code 13 flame loss.
- Rollout or combustion safety trip Blocked flue, heat exchanger cracks, or excessive manifold pressure cause flame rollout and open the rollout switch (code 33), indicating an unsafe condition.
- Reversed polarity or wiring issues Incorrect line voltage wiring, poor ground connection, or loose thermostat wiring can prevent the board from operating or cause intermittent lockouts.
- Blower or control board fault Failed blower motor or internal board failure (codes 61 or 93) stops the furnace from completing its safety checks and starting the heating cycle.
How to Diagnose and Fix
- Read the exact fault code from the LED blink pattern or numeric display on the control board before diagnosing further.
- Check that the furnace has 115 V line voltage at L1 to neutral and L1 to cabinet ground, and verify correct polarity and a solid ground connection.
- If the code is pressure-switch related (45 or 46), inspect the inducer motor operation, check pressure-switch tubing for cracks or blockage, and verify intake and vent piping are clear and properly sloped.
- If the code is ignition or flame related (11, 12, or 13), remove and clean the flame sensor with fine emery cloth or steel wool, inspect the igniter for cracks, verify gas supply is on and inlet pressure is adequate, and check burner ports for debris.
- If the code is limit or overheat (22), replace the air filter, verify all supply registers and return grilles are open and unblocked, check blower operation, and inspect for any airflow restrictions at the blower or heat exchanger.
- If the code is rollout or combustion safety (33), stop immediately and inspect for blocked flue, heat exchanger damage, or excessive gas manifold pressure before attempting restart.
- If no code is present or the furnace does not respond to thermostat calls, disconnect thermostat wiring and jump R to W at the board to test for a thermostat or wiring fault.
- After correcting the root cause, reset the furnace by turning off the thermostat or cycling power briefly, then monitor the next full heating cycle to confirm normal operation.
Parts You Might Need
| Part | Notes |
|---|---|
| Flame sensor rod | Amazon | Inexpensive rod that sits in the burner flame, commonly replaced if cleaning does not restore proper flame sense. |
| Furnace air filter | Amazon | Match the size and MERV rating to your unit, replace every 1–3 months depending on usage and dust load. |
| Pressure switch | Amazon | Diaphragm switch that proves draft, replace if tubing and vent are clear but switch will not close at correct draft pressure. |
Related Error Codes
If your appliance also shows a code on the display, these match this problem:
- Rheem Furnace 2 Flashes error code
- Rheem Furnace 3 Flashes error code
- Rheem Furnace 4 Flashes error code
When to Call a Pro
Call a qualified HVAC technician if you see a rollout fault (33), suspect heat exchanger damage, or lack the tools to measure draft pressure with a magnehelic gauge. Gas valve replacement, igniter testing, combustion analysis, and any work involving gas piping or burner adjustment should be handled by a licensed pro. If you have cleaned the flame sensor and filter, verified gas supply and voltage, and the furnace still locks out on the same code, a technician can pinpoint board faults, intermittent wiring problems, or failed safety components that require specialized diagnostics. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.