Residential No-Heat — Gas Furnace Short Cycling on Ignition
- Interactive decision labs
- Built by working technicians
- Path-based verifiable certificate
- Exam preparation only
- Worldwide online
Residential HVAC Service & Diagnostics
~15 min
intermediate
The call
Homeowner reports intermittent no-heat. The furnace lights, runs for a few seconds, then shuts down and tries again.
- Equipment
- 80% efficiency single-stage gas furnace, hot surface ignition, 11 years old
- Ambient
- 34°F
- Site notes
- Control board flashes a lockout code after three attempts. Filter was replaced last month. Basement installation, PVC-free flue into a masonry chimney.
Decision 1 of 4
You watch a full attempt: inducer starts, pressure switch makes, ignitor glows, burners light cleanly, then the gas valve closes about five seconds later.
What does that sequence tell you first?
Where this lab sits
This is one module lab from Residential HVAC Service & Diagnostics. The full path adds sealed systems, defrost, rack-system orientation and natural-refrigerant safety, each ending in a lab like this one.
Open the full pathNext lab: Residential No-Cool — Split System Not CoolingNext lab: Frozen Coil / Airflow RestrictionNext lab: Superheat / Subcooling DecisionNext lab: Walk-in Cooler Not Holding TemperatureNext lab: Ice Machine Not Making IceNext lab: Reach-in / Prep Table Not Holding TemperatureNext lab: Walk-in Freezer Defrost FaultNext lab: Natural Refrigerant Safe-Handling DecisionNext lab: Rooftop Unit Not CoolingNext lab: RTU Economizer Stuck — No Free CoolingNext lab: RTU High Static — Low Airflow and Poor CoolingNext lab: RTU Gas Heat — No Heat / Ignition Fault