Rheem Furnace Code 0: Standby, No Call for Heat (Meaning and What to Check)

What the code means

Rheem's furnace manuals say that all its furnace controls have a 7-segment diagnostic display, and that during standby with no fault codes present the display reads "0" (zero). When the thermostat calls, the display changes to a letter for the mode: H for gas heat, C for cooling or heat-pump heat, and F for continuous fan. Real faults show as two-digit numbers, displayed one digit at a time (for example 1 then 1 for code 11, failed ignition). Rheem's fault-code label also notes that the control shows its fault history right after power-up, most recent fault first, so numbers that flash briefly when you restore power and then settle on 0 are stored history, not a current problem. So 0 tells you the furnace thinks everything is fine and it's waiting. If you want heat and it never changes to H, the call for heat (the W signal from the thermostat) isn't reaching the control board.

Likely causes

• Nothing is wrong: the thermostat is satisfied and the furnace is idle, which is exactly when 0 is supposed to show
• The thermostat is set to Off, Cool or Fan, or the heat setpoint is at or below room temperature
• Dead thermostat batteries or a blank thermostat screen, so no call is sent
• A smart thermostat that has lost its C-wire power, is in a schedule hold, or is waiting out a compressor/heat delay
• A loose or disconnected thermostat wire, or the thermostat terminal plug not fully seated on the furnace control board
• A broken thermostat cable between the thermostat and the furnace
• Less commonly, a failed thermostat or a control board that isn't reading the call for heat

Safe DIY checks

1. Look at the display while the thermostat is calling for heat. 0 means standby; H means gas heat is on; a two-digit number means a real fault, so look that code up instead.
2. Set the thermostat to Heat and raise the setpoint at least 3 to 5 degrees above room temperature, then give it a few minutes. Many thermostats wait several minutes before starting a cycle.
3. Replace the thermostat batteries if it uses them, and make sure the screen is on. If it's a smart thermostat, check its app or screen for a hold, schedule, or low-power warning.
4. Make sure the furnace power switch is on and the blower door is fully closed and latched. The 0 itself shows the board has power, but a door switch that's barely making contact can cause intermittent problems.
5. With power off at the furnace switch or breaker, check that the thermostat wires are snug at the thermostat and at the furnace control board terminal block, and that the plug is pushed fully onto the board.
6. If you power-cycle the furnace and see a quick series of numbers before it settles on 0, write them down. That's the stored fault history, which can explain an earlier shutdown.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• 24 Carrier Gas Furnace 24
• 33 Carrier Gas Furnace 33
• CL Carrier Gas Furnace CL
• 3 flashes Goodman Gas Furnace 3 flashes
• 4 flashes Goodman Gas Furnace 4 flashes
• 6 flashes Goodman Gas Furnace 6 flashes

All Rheem codes →

Sources

1. Rheem 801T Series furnace manual, diagnostics and fault code display (0 = standby with no fault codes; C, F and H mode letters; how two-digit faults are displayed; fault code list)
2. Rheem 92-103343-01 serial communicating gas furnace fault code label (0 = standby; H furnace heat; fault history shown at power-up, most recent first)
3. All Star Tech HVAC, Rheem furnace diagnostic code 0 (standby, no call, no fault; applies to Rheem/Ruud 802P/80MDP, R95P, R96V)