Rheem Furnace Keeps Shutting Off — What’s Happening
When a Rheem furnace keeps shutting off, the control board has detected a fault and is stopping the heating cycle to protect the equipment. The exact reason depends on the fault code or LED flash pattern displayed on the control, but the most common service-side interpretation is that the furnace is entering lockout from repeated ignition failure, losing flame during the run cycle, or tripping a safety switch like the limit or pressure switch.
Common codes include Code 10 (ignition lockout after multiple failed ignition attempts), Code 12 (low flame sense or weak flame signal), Code 13 (flame lost during the heating cycle), Code 22 (limit switch open from overheating), Code 45 (low pressure open from intake restriction or pressure-switch problem), Code 57 (high pressure open from exhaust restriction), and Code 93 (control board failure). The furnace may shut off within seconds of ignition, after a few minutes of running, or cycle on and off repeatedly.
Most Likely Causes
- Dirty flame sensor rod A contaminated flame sensor cannot conduct the flame rectification current back to the control, causing the board to shut the gas valve off shortly after ignition or trigger a low-flame-sense or flame-lost code.
- Dirty air filter or restricted airflow A clogged filter, blocked return grille, dirty indoor coil, or blower problem starves the heat exchanger of airflow, causing the limit switch to open and shut the furnace down to prevent overheating.
- Blocked intake or exhaust vent Obstructions in the intake or exhaust PVC piping, a blocked pressure-switch sensing tube or port, or vent termination blockage prevent the pressure switch from proving draft or cause it to open mid-cycle.
- Gas supply or gas valve issue A partially closed manual gas shutoff, low manifold pressure, or a failing gas valve can produce a weak or unstable flame that is lost during the cycle or fails to ignite reliably.
- Failed inducer motor or draft problem A weak or failing inducer motor, obstructed inducer blower wheel, or cracked pressure-switch tubing prevents the pressure switch from closing or maintaining draft, stopping the ignition sequence or causing mid-cycle shutdown.
- Polarity, grounding, or wiring fault Reversed line and neutral, missing ground, or incorrect 24 V polarity can prevent the flame sensor from reading correctly or cause the control board to misinterpret signals and shut down the furnace.
- Failed pressure switch, limit switch, or control board A pressure switch that will not close, a limit switch stuck open or tripping prematurely, or an internal control board fault will stop the furnace and may display a specific fault code or prevent ignition entirely.
How to Diagnose and Fix
- Note the exact fault code or LED flash pattern on the control board before resetting the furnace, and confirm whether the furnace is in soft lockout (resets after a few minutes) or hard lockout (requires power interruption).
- Check the air filter and replace it if dirty, then verify that all return and supply registers are open and unobstructed and that the blower is running at the correct speed during the heating call.
- Inspect the flame sensor rod for white or brown contamination, remove it from the burner assembly, and clean the metal rod with fine steel wool or emery cloth until shiny, then reinstall it without touching the cleaned surface.
- Check the inducer motor and pressure switch circuit by listening for the inducer to spin up before ignition, inspecting the clear pressure-switch tubing for cracks or water, and checking the inducer housing and vent terminations for blockage or debris.
- Verify the intake and exhaust vent piping is clear and properly sloped, check that the vent terminations are not blocked by snow, leaves, or nests, and confirm the vent run meets manufacturer length and elbow limits.
- Confirm the manual gas shutoff valve is fully open and that the gas valve position matches the installed gas type (natural or LP), and observe the burner flame for a steady blue flame without yellow tips, lifting, or rollout.
- Check electrical supply and polarity by measuring 115 V between L1 and Neutral at the furnace, then measure L1 to R on the integrated furnace control and verify it reads 96 V (if it reads 144 V, reverse the 24 V and 24 Com wiring to correct transformer phasing).
- If all external causes test good and the furnace still shuts off on the same code, test or replace the component indicated by the fault code (flame sensor, pressure switch, limit switch, inducer motor, gas valve, or control board) or call a qualified technician for combustion analysis and further diagnostics.
Parts You Might Need
| Part | Notes |
|---|---|
| Rheem flame sensor | Amazon | Verify the sensor bracket and rod length match your model before ordering. |
| Rheem pressure switch | Amazon | Match the switch closing pressure (in inches water column) stamped on the original switch. |
| Rheem integrated furnace control board | Amazon | Confirm the exact model and revision number; many Rheem boards are model-specific and not interchangeable. |
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 are uncomfortable working with gas appliances, if the furnace continues to shut off after cleaning the flame sensor and changing the filter, if the fault code points to a gas valve or control board fault, or if combustion analysis or pressure-switch testing with a manometer is required. A technician can verify manifold gas pressure (Rheem specifies the inducer should produce at least 0.1 inch water column beyond the pressure-switch closing setpoint), check flame rectification current, test safety-switch circuits with a multimeter, and diagnose internal board faults or failed components that require part replacement. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.