There is a wrinkle with Rheem that catches people out before they ever read a code: Rheem puts its badge on furnaces, air conditioners, water heaters and pool heaters, and the code tables are completely different between them. If the appliance blinking at you is a tall cylinder or a wall-hung box feeding hot water, you are looking at a water heater and this page will send you down the wrong path. This guide covers the forced-air gas furnace only.
On the furnace, the diagnostic light sits on the integrated control behind the lower access panel, visible through a small window on most models so you can read it without opening anything. Ruud furnaces are the same machine under a different name and read identically. On variable-speed models a two-character alphanumeric display replaces blink counting, which is easier to read accurately and worth using when you have it.
The usual caveat applies with more force here than most, because Rheem has revised its furnace controls several times: the legend printed on the label inside your blower compartment governs, and where this page and that label disagree, the label is correct. What does not change between generations is what each family of fault physically means, which is what the table explains. If reading it in a cold house is not appealing, our Rheem service page lists what we carry and ZK Mechanical will walk through it with you on the phone.
Rheem furnace codes, decoded
| Code | What it means | Safe? | Try first | When to call |
|---|---|---|---|---|
Steady on | Control powered, no fault stored, waiting for or answering a call for heat. | keep running | Nothing to do. | Only if there is no heat despite a normal light. |
No light at all | The control has lost power, or its fuse has opened. | turn it off | Confirm the service switch is on, the breaker has not tripped, and the panel is fully closed. | When those three check out and the light is still dead. |
Ignition lockout | The furnace ran through its permitted trials for ignition without establishing flame, and has locked itself out. | turn it off | A single reset is reasonable. Most often the flame sensor is dirty or the igniter has failed. | On the second lockout. Repeating trials is how a small part failure becomes a bigger one. |
Pressure switch open | The inducer is running but the switch never proved draft, so the control will not open the gas valve. | turn it off | Walk outside and clear snow, ice or nesting from the intake and exhaust terminals. | If the terminals are clear — the condensate drain, the hose or the inducer is the next suspect. |
Pressure switch closed at rest | The switch reads closed before the inducer starts, which the control treats as a failed safety. | turn it off | Nothing at home. A switch that will not open is a failed switch or a wiring fault. | Yes. Draft proving is a safety function and the furnace correctly refuses to fire without it. |
Open limit | The furnace exceeded its temperature limit and the burner was shut down to protect the heat exchanger. | turn it off | Fit a clean filter and open closed registers before restarting. | If it repeats with good airflow. Chronic overheating shortens heat exchanger life. |
Rollout tripped | Flame or combustion gas escaped the burner compartment and the rollout switch cut the gas. | call now | None. Do not press the reset button on the rollout switch. | Immediately, and leave the furnace off until it has been inspected. |
Flame with valve closed | The control senses flame when the gas valve should be shut. | call now | Shut the furnace off at the service switch. | Immediately. Either the valve is leaking through or the sensing circuit is lying. |
Reversed polarity or no ground | Line voltage is landed backwards, or the ground is missing, so flame sensing cannot work. | turn it off | Nothing safe. This is a receptacle or panel issue for an electrician or the gas tech. | Yes, and mention it up front so the right meter comes in the door. |
Low flame current | Flame is being detected, but the signal is marginal and will eventually drop out mid-cycle. | keep running | Book maintenance while the furnace is still working. | Soon rather than urgently. This is the early warning, not the failure. |
Blower or motor fault | The control cannot confirm the circulating blower is doing what it was told. | turn it off | Listen for a hum without air movement and note it for the technician. | Yes. A furnace firing without reliable circulation will trip its limit repeatedly. |
Inducer fault | The combustion-air inducer is turning, but not at the rate the control expects before it will let the gas valve open. | turn it off | Nothing at home. Note any new rattle or grinding and when it started. | Yes. An inducer on its way out shows up as pressure switch faults soon after. |
The codes that usually mean a real repair
Lockouts that follow the cold
A Rheem that lights fine in November and starts locking out in January is usually telling you about marginal flame sensing rather than a sudden failure. Colder air is denser, combustion shifts slightly, and a sensor already reading low falls under the threshold. The fix is at the inexpensive end of the $150–$450 range most common repairs fall into, and it is far cheaper than the emergency call it prevents.
Pressure switch faults on high-efficiency models
Condensing Rheem furnaces drain water as they run, and in this climate the two things that stop them are a frozen or blocked vent terminal outside and a plugged condensate trap inside. Both present as a pressure switch fault. We clear the drain, verify the draft with a meter rather than by ear, and check the vent termination clearances outside — the escarpment and lakeshore both drift snow against sidewall vents in ways an installer in a milder city never has to think about.
When the code points at the heat exchanger
Rollout and repeated limit trips are the two codes that make us look hard at the heat exchanger before quoting anything. If it is cracked on a furnace of a certain age, replacement usually beats repair on the math — a new high-efficiency furnace installs in the $4,500–$7,500 range and carries a fresh warranty. We explain how a cracked heat exchanger is confirmed rather than asking you to take it on faith.
Licensed technicians, trucks stocked with Rheem parts, no overtime charges.
Common questions
My Rheem water heater is flashing. Does this page apply?
No. Rheem uses entirely separate diagnostic tables for water heaters, and a code that means one thing on a furnace means something unrelated on a tank or tankless unit. Check what the blinking appliance actually is first — if it heats water rather than air, you need the water heater literature, not this page.
Is a Ruud furnace the same as a Rheem?
For diagnostic purposes, yes. Ruud and Rheem furnaces come off the same platform with different badging, and the control and its legend behave the same way. Parts cross over as well, which is useful when something needs sourcing quickly.
The code cleared itself. Should I still call?
It depends which one. A lockout that cleared after a power cycle is worth watching; a rollout, a flame-with-valve-closed or a polarity fault is worth a visit even if the light has gone quiet, because those three are safety devices reporting that something happened. Codes stored in memory are also readable by a technician after they stop displaying.
The house is cold and my Rheem is locked out. What should I do while I wait?
Set the thermostat to off so the control stops retrying, and leave the furnace switch off entirely if the code involved flame, venting or a rollout. Close doors on rooms nobody is using, keep a trickle running at taps on exterior walls, and check that every CO alarm still works. Our emergency line is answered around the clock with no overtime, weekend or holiday surcharge — 24/7 emergency service sets out what counts as one.
Does a code mean the board needs replacing?
Rarely. The board is the messenger for almost every code in the table — it is reporting a switch, a sensor, a motor or a wiring fault it detected elsewhere. Boards do fail, but a board quoted without anyone measuring the flame signal, the pressure switch circuit or the limit string deserves a second opinion.