Troubleshooting a Low Voltage Short

Indications: A Blown Low Voltage Fuse or a Bad Transformer

1. Prepare to Diagnose Short

• Turn off line-voltage power to the air handler or furnace.
• Install a resettable low voltage breaker (if not already on the transformer).
• At the furnace or air handler, disconnect the low voltage wiring to the thermostat. (Leave all other low voltage wiring connected.)
• Turn line-voltage power back on.
  • If low voltage breaker trips - go to Immediate Short.
  • No short, proceed with Isolating the Short.

2a. Isolating the Short (Heat Pump)

Use a jumper wire to test each output wire at the air handler as follows:

• Connect R to G = Energize fan relay.
  • No short - proceed to next test.
  • A short would indicate a bad fan relay or control board.
• Connect R to O = Energize reversing valve.
  • No short - proceed to next test.
  • A short would indicate shorted wiring between the air handler & reversing valve or a shorted solenoid coil.
• Connect R to Y = Energize compressor. Wait until compressor contactor energizes.
  • No short - proceed to next test.
  • A short would indicate shorted wiring between the air handler & contactor or a shorted contactor coil.
• R to W = Energize heat (aux heat on heat pumps). 
  • No short - proceed to next test.
  • A short could indicate a shorted limit, shorted heat contactor, shorted heat relays or sequencers, or miswired heaters.
• If no short, go to Test Thermostat.

2b. Isolating the Short (Furnace & A/C)

Use a jumper wire to test each output wire at the furnace or air handler as follows:

• Connect R to G = Energize fan relay.
  • No short - proceed to next test.
  • A short would indicate a bad fan relay or control board.
• Connect R to Y = Energize compressor. Wait until compressor contactor energizes.
  • No short - proceed to next test.
  • A short would indicate shorted wiring between the air handler & contactor or a shorted contactor coil.
• R to W = Energize heat.
  • No short - proceed to next test.
  • A short could indicate a shorted limit, shorted heat contactor, shorted heat relays or sequencers, or miswired heaters.
• If no short, go to Test Thermostat.

3. Test Thermostat (Heat Pump)

• Turn off line-voltage power to the air handler.
• At the air handler terminal connections, re-connect R and C to the thermostat.
• At the thermostat, set the System switch to "Off" and the Fan switch to "Auto".
• Turn on line-voltage power to the air handler.
  • No short - proceed to next step.
  • An immediate short indicates either a bad thermostat or shorted thermostat wire.
    • At the thermostat, disconnect R.
    • Start over at Step 1.
    • No short = Bad thermostat.
    • Short = Shorted stat wire.
• At the air handler, jump from R on the air handler to each thermostat wire from the stat:
  • R on the air handler to G going to the stat.
  • R on the air handler to O going to the stat.
  • R on the air handler to Y going to the stat.
  • R on the air handler to W going to the stat.
  • R on the air handler to L going to the stat.