INTRODUCTION:
When your Rheem furnace stops working and the status light starts flashing a coded pattern it’s trying to tell you exactly what’s wrong. Rheem furnaces use a built-in diagnostic system that communicates faults through LED flash codes — and knowing how to read these codes is the difference between a quick DIY fix and an unnecessary service call.
Rheem is one of the most widely installed furnace brands in North America and their error code system is one of the most straightforward to understand. This complete guide covers every major Rheem furnace error code, what each one means, and the exact steps to fix it — starting with the most common codes homeowners encounter.
H2: How to Read Rheem Furnace Error Codes
Rheem furnaces communicate faults through a status LED light located on the control board — visible through a small window on the furnace door panel. The light blinks in a specific pattern of flashes followed by a pause.
To read the code count the number of flashes before the pause. For example if the light flashes 3 times pauses then flashes 3 times again your code is 3-3. Some newer Rheem models display a digital code on a small screen rather than using flash patterns.
Most Rheem furnaces also have a fault code reference sticker inside the furnace door. Check this first as it lists the specific codes for your exact model — some codes vary slightly between model generations.
To read your code accurately: Turn your thermostat off and wait 30 seconds. Turn it back on and watch the LED carefully as the furnace attempts to start. Use your phone to record a short video of the LED — playing it back slowly makes counting flash patterns much easier than trying to count in real time.
H2: Rheem Furnace Error Code 1 Flash — System Lockout
What it means: One flash indicates your Rheem furnace has gone into system lockout after repeated failed ignition attempts. The furnace tried to light the burners multiple times and failed — triggering a safety lockout to prevent unburned gas from accumulating.
Most common causes: A dirty or failed ignitor is the leading cause of this code. The hot surface ignitor weakens over time and eventually fails to reach the temperature needed to ignite the gas. Other causes include a faulty flame sensor, low gas pressure, or a failed gas valve.
How to fix it: Reset the furnace by turning the power switch off for 30 seconds and back on. Watch the startup sequence carefully. If you hear the inducer start and the gas valve click but no ignition occurs the ignitor is the most likely culprit.
A replacement Rheem hot surface ignitor costs $15 to $40 on Amazon and is a straightforward DIY replacement. If the ignitor glows orange but the flame won’t stay lit clean the flame sensor — a simple 10 minute job using fine steel wool.
H2: Rheem Furnace Error Code 2 Flashes — Pressure Switch Stuck Open
What it means: Two flashes means the pressure switch failed to close during startup. The pressure switch verifies that the inducer motor is creating proper draft before allowing the gas valve to open — a critical safety function that ensures combustion gases vent properly.
Most common causes: A blocked condensate drain line is the most common cause on high efficiency Rheem furnaces. When the drain backs up water can fill the pressure switch port and prevent it from reading correctly. Other causes include a cracked pressure switch hose, a failed inducer motor, or an actual failed pressure switch.
How to fix it: Start with the condensate drain — pour one cup of white distilled vinegar into the drain line access point to clear algae and minor blockages. Check the small rubber hose connecting the pressure switch to the inducer housing for cracks, kinks, or disconnections.
Listen for the inducer motor during startup — it should run continuously from the moment the furnace attempts to start. If the inducer sounds weak or doesn’t start the motor may be failing and requires professional attention.
Pressure switch replacement costs $15 to $50 and is a DIY friendly repair for most homeowners.
H2: Rheem Furnace Error Code 3 Flashes — Pressure Switch Stuck Closed
What it means: Three flashes is the opposite problem from code 2 — the pressure switch is closed when it should be open. The control board detected that the switch was already in the closed position before the inducer motor started.
Most common causes: A stuck or shorted pressure switch, water inside the pressure switch from a backed up condensate drain, or a short circuit in the pressure switch wiring.
How to fix it: Disconnect the small rubber hose from the pressure switch port and check for water inside. Water in the hose indicates a condensate drain problem — clear the drain and dry the hose before reconnecting.
If no water is present the pressure switch itself is likely stuck closed and needs replacement. As noted above pressure switches are inexpensive and DIY replaceable.
H2: Rheem Furnace Error Code 4 Flashes — Open High Limit Device
What it means: Four flashes is one of the most common Rheem furnace error codes. It means the high limit switch — a safety device that monitors heat exchanger temperature — has opened because the furnace overheated.
Most common causes: A clogged air filter is responsible for the majority of code 4 calls. Restricted airflow causes the heat exchanger to overheat and the limit switch to trip. Other causes include blocked vents, a dirty or slow blower motor, or in serious cases a cracked heat exchanger.
How to fix it: Replace your air filter immediately — this resolves code 4 in most cases. Check that every supply and return vent in your home is fully open and unobstructed. Reset the furnace and test.
If code 4 returns after filter replacement and vent check have a technician inspect your blower motor and heat exchanger. A cracked heat exchanger is a serious safety hazard that allows combustion gases including carbon monoxide to mix with your home’s air supply.
H2: Rheem Furnace Error Code 5 Flashes — Flame Sensed Without Gas Valve
What it means: Five flashes means the flame sensor is detecting a flame when the gas valve hasn’t been commanded to open. This is a safety related code that causes immediate shutdown — the control board shuts everything down when it detects a flame that shouldn’t be there.
Most common causes: A faulty flame sensor sending false signals is the most common cause. A failed gas valve that isn’t fully closing is less common but also possible.
How to fix it: Clean the flame sensor first — this is a free fix that resolves the majority of code 5 cases. Turn off all power to the furnace. Locate the flame sensor — a small metal rod positioned in the burner flame path with a single wire attached. Remove the retaining screw and gently rub the metal rod with fine steel wool to remove the white oxide coating. Reinstall and test.
If cleaning doesn’t resolve the code the gas valve or control board may need professional inspection.
H2: Rheem Furnace Error Code 6 Flashes — Rollout Switch Open
What it means: Six flashes indicates the rollout switch has tripped. The rollout switch is a safety device located near the burner compartment that detects if flames are rolling out of the heat exchanger — a dangerous condition that indicates a serious problem.
Most common causes: A blocked heat exchanger, a cracked heat exchanger, or a blocked flue vent that causes combustion products to back up into the burner compartment. In some cases a very dirty burner can cause abnormal flame patterns that trigger the rollout switch.
How to fix it: Do not simply reset the rollout switch and continue operating the furnace without identifying the root cause. A tripped rollout switch indicates a potentially dangerous condition. Call a licensed HVAC technician to inspect the heat exchanger and flue system before operating the furnace again.
The rollout switch itself can be reset by pressing the small button on the switch — but only after the underlying cause has been identified and corrected.
H2: Rheem Furnace Error Code 7 Flashes — Low Flame Signal
What it means: Seven flashes means the furnace ignited successfully but the flame sensor is detecting an abnormally weak flame signal. The control board isn’t confident the burner is operating correctly and shuts down as a precaution.
Most common causes: A dirty flame sensor is the most common cause — even a thin layer of oxidation on the sensor rod dramatically reduces the signal it sends to the control board. Other causes include low gas pressure, partially clogged burners, or a weakening flame sensor that needs replacement.
How to fix it: Clean the flame sensor as described in the code 5 section. This free fix resolves code 7 in the vast majority of cases. If cleaning doesn’t help check that the gas shutoff valve at the furnace is fully open and that other gas appliances in your home are working normally — indicating adequate gas pressure.
Burner cleaning with compressed air is the next step if the flame sensor and gas pressure check out normally.
H2: Rheem Furnace Error Code 8 Flashes — Low Flame Signal with Retries
What it means: Eight flashes is similar to code 7 but indicates the furnace detected a low flame signal and attempted multiple retries before locking out. The system tried to establish a stable flame several times and failed each time.
Most common causes: The same causes as code 7 — dirty flame sensor being the most common — but potentially more severe. A flame sensor that cleans up temporarily but keeps getting coated may indicate an underlying combustion issue.
How to fix it: Clean the flame sensor thoroughly and test. If code 8 returns quickly after cleaning consider replacing the flame sensor entirely — a new Rheem flame sensor costs $10 to $25 on Amazon and takes about 15 minutes to replace.
If a new sensor doesn’t resolve the issue low gas pressure or a partially blocked heat exchanger requires professional diagnosis.
H2: Rheem Furnace Error Code 9 Flashes — Ignition Lockout
What it means: Nine flashes means the furnace attempted to ignite the burners the maximum number of times allowed and failed each attempt. The system has entered hard lockout mode and will not attempt another ignition cycle until manually reset.
Most common causes: A failed hot surface ignitor is the most common cause followed by a faulty flame sensor, low gas pressure, and a failed gas valve.
How to fix it: Reset the furnace and watch the complete startup cycle carefully. Listen for these sounds in order — inducer motor starting, a brief pause, a click from the gas valve opening, then ignition. Identifying where in the sequence the failure occurs points you to the failed component.
No glow from the ignitor — the ignitor has failed and needs replacement. Ignitor glows but no flame — gas isn’t reaching the burner. Check gas supply and gas valve. Flame lights but goes out immediately — flame sensor needs cleaning or replacement.
H2: Rheem Furnace Continuous Flashing — Normal Operation
If your Rheem furnace status light is flashing continuously in a steady on-off pattern rather than a coded sequence this indicates normal operation — not a fault condition. A continuously blinking light simply means the furnace is powered on and the control board is functioning normally.
Only a coded pattern with distinct pauses between flash groups indicates a specific fault code.
H2: How to Reset Your Rheem Furnace
Regardless of which error code you’re dealing with the reset procedure is the same for all Rheem furnaces:
Locate the power switch on or near the furnace — it looks like a standard light switch. Turn it to the off position. Wait a full 30 seconds. Turn it back on. Go to your thermostat and call for heat by setting it at least 5 degrees above the current room temperature.
Watch and listen as the furnace attempts to start. If the same error code returns the underlying problem hasn’t been resolved and needs diagnosis.
Never reset your furnace more than 3 times in a row — repeated resets without resolving the underlying problem can cause additional damage and in the case of gas related faults can be dangerous.
H2: Rheem Furnace Maintenance to Prevent Error Codes
Most Rheem furnace error codes are preventable with regular maintenance:
Replace your air filter every 60 to 90 days — clogged filters cause the majority of high limit and lockout codes.
Clean your flame sensor annually — a 10 minute task that prevents the majority of ignition and flame signal codes.
Keep your condensate drain clear — pour vinegar down the line every 90 days to prevent the pressure switch codes caused by drain backups.
Schedule a professional tune up every 1 to 2 years — a technician can catch weakening ignitors, capacitors, and other components before they fail and cause error codes.
H2: When to Call a Professional for Rheem Furnace Repairs
Handle these yourself:
- Code 1 and 9 — clean or replace the ignitor
- Code 2 and 3 — check condensate drain and pressure switch hose
- Code 4 — replace air filter first
- Code 5, 7, and 8 — clean the flame sensor
Call a licensed HVAC technician for:
- Code 6 — rollout switch tripped — potential heat exchanger crack
- Any code involving gas valve diagnosis
- Any situation where you smell gas — leave immediately and call your gas company
- Codes that return immediately after the recommended DIY fix
- Any furnace making grinding, screeching, or banging sounds
H2: Frequently Asked Questions
Q: Where is the error code light on my Rheem furnace? The status LED is on the control board inside the furnace. Most Rheem models have a small rectangular window in the lower furnace door through which you can see the LED without opening the furnace. If you can’t find it check your furnace manual or look up your model number online.
Q: How do I reset my Rheem furnace after an error code? Turn the furnace power switch off for 30 seconds then back on. This clears the lockout and allows a new startup attempt. If the same code appears again the underlying problem needs to be addressed before the furnace will operate normally.
Q: My Rheem furnace is flashing but I can’t figure out the pattern — what should I do? Use your phone to record a short video of the LED. Play it back at normal or slow speed and count the flashes before and after each pause. The pattern will be much clearer on video than trying to count in real time.
Q: How long does a Rheem furnace lockout last? Most Rheem furnace lockouts require a manual reset — they don’t clear automatically after a set time. Turn the power switch off for 30 seconds and back on to reset.
Q: Is it safe to keep resetting my Rheem furnace? Resetting once or twice to confirm an error code is acceptable. However repeatedly resetting without fixing the underlying problem is not safe — especially for gas related faults. If your furnace fails to start after 2 to 3 resets stop resetting and diagnose the root cause.
Q: How much does a Rheem furnace repair cost? Simple repairs like flame sensor cleaning cost nothing. Ignitor replacement costs $50 to $150 with labor. Control board replacement costs $300 to $600. Heat exchanger issues often make replacement more economical than repair on older units.
