HVACopediaAll brands · General troubleshooting
No heat — low-voltage stat diagnostic (16 steps)
What is going on
Possible causes and checks for: no heat — low-voltage stat diagnostic (16 steps).
Likely causes
- Power off
- Jumper R–W at furnace board
- Power on, door switch depressed, wait ≤
- Min. Furnace starts → problem is stat/wiring
- Doesn't → furnace. Remove jumper
- At stat subbase read R–W = 25–27 Vac. No voltage → wire fault: read R–G and R–Y
- If none read source, R wire is bad
- If they do, W wire is bad
- Swap to a spare conductor
- Still nothing anywhere → whole cable broken, replace. Voltage present → jumper R–W at subbase, heat starts within
- Min → stat is bad
- Check batteries first on electronic stats
- Replace mercury stats with subbase. Remove all jumpers. p.991 [965]
More general guides
Take it to the job
All guides, error codes and tips work offline in the HVACopedia app.
Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.