How to Fix / Troubleshooting

Safety first: This involves line-voltage wiring. If you are not qualified, contact an electrician.

Steps:

Turn off power at the breaker panel.
Remove the furnace electrical junction box cover and verify that the incoming hot, neutral, and ground are correctly connected to the CTC furnace wiring harness per the wiring diagram.
Use a plug-in outlet tester or multimeter at the supply receptacle (if cord-connected) to confirm correct polarity and ground.
Correct any reversed hot/neutral or missing ground conditions at the panel or junction box.
After corrections, restore power and clear the error by cycling power to the furnace.

Note: Persistent polarity errors may indicate internal board issues; the CTC control board may need replacement.

Furnace control indicates reversed or missing line polarity / improper grounding.

Possible Causes