E7
American Standard High/Low Voltage Supply Fault to Thermostat

Thermostat displays E7 and system may lock out heating or cooling

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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

Incorrect transformer voltage, Loose R or C wire at thermostat, Short between R and C or R and another terminal, Failing 24V transformer in air handler or furnace

How to Fix / Troubleshooting

Safety: Turn off power to the furnace/air handler at the breaker. 24V circuits can still arc and damage components if shorted.

  • Check thermostat wiring: Remove the thermostat and inspect the R and C terminals. Ensure wires are tight and no copper strands are touching adjacent terminals.
  • Measure control voltage: With a multimeter (and power restored), measure AC voltage between R and C at the thermostat sub-base. It should typically be 24–28 VAC. If it is much lower or fluctuating, there may be a transformer or wiring issue.
  • Inspect at air handler/furnace: Turn power off again, remove the blower door, and inspect the low-voltage terminal strip. Tighten any loose R and C connections and look for burned or discolored wires.
  • Check for shorts: Look for thermostat cable runs that may be pinched by sheet metal or staples. Repair any damaged sections.

Note: If the transformer is failing or repeatedly blowing fuses, a licensed HVAC technician should replace it and locate the underlying short.

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Repair Difficulty

Hard 4/5

Required Part

24V control transformer / R-C low-voltage wiring
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