Rheem Furnace Blink Codes: 2–9 Flashes — Complete Diagnosis Guide

Common Symptoms

• LED on the control board blinking a repeated flash-pause-flash pattern
• Furnace not producing heat or locking out
• Furnace lights briefly then shuts off within 1–3 seconds
• Inducer runs but ignition sequence never completes
• Furnace tries to start multiple times and then stops responding

Most Likely Causes

1. 1

   2 Flashes — System Lockout (Retry or Recycle Lockout)

   The furnace control board attempted ignition the maximum number of times without proving a stable flame and entered hard lockout. The furnace will not retry until manually reset. The 2-flash code itself does not identify which component failed — it tells you the ignition sequence failed. Diagnose the ignition system (igniter, flame sensor, gas pressure, gas valve) to find the underlying fault before resetting, or the lockout will repeat immediately.

2. 2

   3 Flashes — Pressure Switch Fault

   The draft inducer pressure switch failed to close during startup, or closed at the wrong time. Causes: a blocked or cracked pressure hose, an inducer motor not generating sufficient draft (failed motor, dirty housing, cracked wheel), a blocked flue pipe or condensate drain on 90+ models, or a defective pressure switch. On Rheem Classic and Classic Plus 90+ efficiency models, the secondary heat exchanger condensate drain is a frequent culprit for 3-flash codes — a clogged drain traps condensate and prevents the inducer from building pressure.

3. 3

   4 Flashes — Open Temperature Limit Device

   A high-temperature limit switch or rollout switch has opened. The furnace shuts off and will not restart until the heat exchanger cools and the switch resets automatically (or is manually reset if it's a rollout switch). Primary cause: clogged air filter restricting airflow. Secondary causes: blocked return air, failed blower motor, closed supply registers. If the limit trips repeatedly, check the blower motor capacitor and motor amp draw — an underperforming blower causes repetitive 4-flash codes even with a clean filter.

4. 4

   6 Flashes — Open Flame Sensor / Flame Sense Failure

   The flame sensor circuit is open or the sensor cannot detect the flame during the ignition trial. On Rheem boards, code 6 indicates either an open-circuit flame sensor (wiring issue or failed sensor) OR a coated sensor with too-low flame rectification current. If the furnace lights visibly through the sight glass but shuts off after 1–3 seconds, the sensor is detecting flame but the signal is too weak — cleaning will fix it. If the furnace never lights and code 6 appears, check the sensor wiring first.

5. 5

   7 Flashes — Gas Valve Circuit Fault

   The control board detected an electrical fault in the gas valve circuit. Causes: the gas valve coil is open or shorted (test resistance across valve terminals — healthy coils read 20–80 ohms), a wiring harness connector to the gas valve has failed, or the valve has an internal solenoid fault. Code 7 can also appear if the 24VAC signal to the gas valve is present but the valve does not open mechanically — confirmed by measuring gas pressure at the manifold pressure port with a manometer during an ignition attempt.

6. 6

   9 Flashes — Reversed Line Voltage Polarity

   The power supply hot and neutral wires feeding the furnace are reversed. This is a wiring fault — check the furnace power switch wiring and the circuit breaker connections. The furnace will not operate safely with reversed polarity. Code 9 is often introduced after a homeowner replaces the furnace on/off switch or reroutes wiring. Confirm with a non-contact voltage tester at the furnace disconnect to identify the hot and neutral legs.

Quick DIY Checks

1. 1Count the blink code accurately: open the lower furnace door or look through the observation window to see the control board LED. Count the flashes in a single sequence before the pause. The LED blinks, then pauses approximately 2–3 seconds, then repeats the same count. Count through at least two full cycles to confirm the number. A slow single blink repeated continuously (approximately one flash per second) means normal operation. A fast continuous blink means control board fault. A steady 'on' with no blinking can also indicate certain fault states — check the sticker on the inside of the furnace door for a model-specific code table.
2. 2For a 3-flash pressure switch code: locate the small rubber pressure hose (usually 1/4 inch tubing) running from the inducer motor housing (or secondary heat exchanger on 90+ models) to the pressure switch. Trace the hose its full length and check for kinks, cracks, or white residue from condensate deposits that has hardened inside the tube. Disconnect both ends of the hose and blow through it firmly to clear any blockage. On Rheem 90+ efficiency models (Classic Plus, Prestige series), also check the secondary condensate drain line — locate the drain port on the secondary heat exchanger collector box and verify water is draining freely. A blocked secondary drain causes the collector box to fill with condensate, which floods back into the inducer housing and prevents proper pressure switch operation.
3. 3For a 4-flash high limit code: replace the air filter immediately — this is the single most effective step for 4-flash codes and solves the problem in the majority of cases. Pull the filter from the return air slot in the furnace cabinet or from the wall/ceiling return air grille. A filter that is gray or matted even slightly can restrict airflow enough to trip the limit. After replacing the filter, let the furnace sit for 20 minutes with power on (heat demand off) to allow the heat exchanger to cool and the limit switch to auto-reset. Then call for heat. If 4 flashes return within one heat cycle with a fresh filter, inspect the blower motor: with the furnace running in heat mode, the blower should start within 30–60 seconds of the burners lighting and should run continuously until the heat demand ends. A blower that starts late, runs slowly, or does not start at all indicates a capacitor or motor failure — test the run capacitor with a multimeter in capacitance mode.

1. 4For a 6-flash flame sense code: turn off power and gas. Locate the flame sensor rod in the burner assembly — a thin metal rod (approximately 2–4 inches long) mounted with a ceramic insulator and a single wire connector. Remove the sensor (one screw) and inspect the metal rod tip. A white, gray, or brownish oxide coating on the metal is normal buildup that reduces the flame rectification signal. Buff the rod tip lightly with 120-grit emery cloth or fine steel wool until the metal is clean and bright. Do not buff the ceramic insulator — only the metal portion. Reinstall, restore power and gas, and run a heat cycle. Watch through the sight glass — the furnace should light and remain lit. If code 6 returns after cleaning, check the single wire from the sensor to the board for a loose or corroded connector. A flame sensor that has been cleaned three or more times may have too much material removed and needs replacement.
2. 5For a 7-flash gas valve code: turn off power. Locate the gas valve body (the large valve mounted on the gas manifold assembly inside the furnace cabinet). Find the two wire connectors to the valve solenoid coils. Disconnect them and measure resistance with a multimeter set to ohms: a typical Rheem gas valve solenoid reads 20–80 ohms. OL (open circuit) means the coil has burned out — the valve needs replacement. A reading near 0 ohms means the coil has shorted. If the coil resistance is within range, the fault may be a wiring issue — inspect the connector and wire harness between the board and valve for corrosion or broken pins. Also verify the gas supply valve upstream of the furnace is fully open. A partially open manual gas shutoff valve can produce sufficient pressure at rest but drop too low during a heat call to open the furnace gas valve fully.
3. 6Lockout reset procedure for Rheem/Ruud 2-flash lockout: set the thermostat to OFF or lower the set point below room temperature. Locate the furnace power switch (looks like a wall light switch, typically on the wall or gas line near the furnace) and flip it to OFF. Wait 30 full seconds. Restore the power switch, then set the thermostat to HEAT at least 5°F above room temperature. Watch and listen — the inducer fan should spin up first (within 30 seconds), followed by the igniter glowing (visible through the sight glass), then ignition within 60–90 seconds of a heat call. If the furnace attempts ignition and goes to 2 flashes again within one cycle, the underlying ignition fault was not corrected. Continue diagnosing the igniter, flame sensor, and gas supply before resetting again.

Repair vs Replace

Recommended Tools & Parts

• Rheem Ruud Hot Surface Igniter (Silicon Nitride)

  Replacement silicon nitride igniter for Rheem and Ruud gas furnaces. Fixes 2-flash lockout caused by a failed igniter. Common Rheem part numbers include 62-23543-01, 62-22868-92, and SP11172. Verify 120V vs 24V voltage for your model before ordering.

  $25–$65

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• Rheem Ruud Flame Sensor Rod

  Replacement flame sensor for Rheem and Ruud furnaces. Fixes 6-flash flame sense code when cleaning fails to restore adequate flame signal. Match the mounting bracket and wire length to your existing sensor.

  $15–$35

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• Rheem Ruud Draft Inducer Pressure Switch

  Replacement pressure switch for Rheem and Ruud furnaces. Fixes 3-flash pressure switch code when hose is clear. Match the wye-number water column rating marked on your existing switch.

  $20–$55

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• Rheem Ruud High Limit Switch

  Replacement high-temperature limit switch for Rheem and Ruud furnaces. Fixes 4-flash open limit code when filter and blower have been verified. Match the temperature rating and mounting diameter to your existing switch.

  $15–$40

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Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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