12
Rheem
Low 24V Control Voltage
Status LED flashes 12 times; furnace may intermittently fail to start
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Weak or failing 24V transformer, Short in thermostat wiring, Loose low-voltage connections, Oversized load on 24V circuit (add-on accessories)
How to Fix / Troubleshooting
Safety first: Turn off high-voltage power before working near the transformer or control wiring.
- Measure 24V output: With a multimeter, measure between R and C on the control board. Voltage should be around 24–28 VAC. If low or fluctuating, suspect the transformer or a short.
- Inspect thermostat cable: Look for staples through the cable, damaged insulation, or bare wires touching metal. Repair or replace damaged sections.
- Disconnect accessories: Temporarily disconnect humidifiers, zone boards, or Wi-Fi thermostats from the R/C circuit to see if voltage stabilizes.
- Replace transformer: If the transformer is hot, humming loudly, or output is low with no shorts found, replace with a Rheem-compatible 24V transformer of equal VA rating.
Do not upsize the transformer VA rating without confirming wire sizes and connected loads.
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Repair Difficulty
Medium
3/5
Required Part
24V Control Transformer
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