Trane: Weak or Lost Flame Signal
What this fault means
The furnace lit but the flame-sensing rod measured too little current, so the board closed the gas valve. The single most common furnace fault we see each winter in BC.
Likely causes
- Oxidized or dirty flame sensor rod
- Poor burner grounding
- Cracked sensor porcelain
- Marginal gas/air mixture
What you can safely check
- Replace a dirty air filter — restricted airflow causes a surprising share of furnace faults.
- Confirm registers and the outdoor unit are clear of snow, leaves and debris.
- Cycle power at the switch once and let the system retry.
Call a technician — don't keep resetting this
Weak or Lost Flame Signal is a high-severity fault. Repeated resets of a safety lockout can mask a serious problem such as a cracked heat exchanger (a carbon-monoxide risk) or a failing compressor. If you smell gas, leave and call FortisBC at 1-800-663-9911 or 911 first.
Related Trane faults
Trane furnace showing Weak or Lost Flame Signal?
CANRO Heating & Cooling — same-day diagnosis across Greater Vancouver. Call 604-359-1081.