Forced Air Furnace: No-Heat
Verify Control Voltage Logic Tree
* These diagnostic trees are primarily for training purposes and may not cover all possible scenarios.
1) 24 volts between R and C terminal at the furnace control board?
Control Voltage Verified.
2) Is power on to the unit (breaker on, remote switch on, furnace door switch pressed in)?
Restore power. Return to Step 1.
3) Is the 3 or 5 amp fuse on the control board blown?
Short circuit occurred in low voltage wiring. Find and repair, then replace fuse.
Note: If a thermostat was recently replaced while power was on, a short may have occurred but may no longer exist. Use a 3 or 5 A fuse tester before installing a new fuse.
4) Is 24 volts present from either side of the fuse to the Common (C) terminal on the control board?
5) Check if a flame roll-out or high thermal limit switch is open.
These are often wired after the fuse but before the R terminal. Some must be manually reset.
These are often wired after the fuse but before the R terminal. Some must be manually reset.
Address the issue tripping the safety. Reset/replace the safety as appropriate.
6) Measure for 24 V on the secondary (low voltage) side of the transformer.
Measure at the transformer or where the secondary wires land on the control board.
Measure at the transformer or where the secondary wires land on the control board.
Check for loose/broken connections. Possible open board trace (bad control board). Power off and inspect the back of the board for burn marks/damage.
7) Verify 120 V on the primary (120 V) side of the transformer.
Measure between the transformer’s 120 V (hot) and Neutral wires coming from the control board. High voltage — do not use the low-voltage C terminal for this test.
Measure between the transformer’s 120 V (hot) and Neutral wires coming from the control board. High voltage — do not use the low-voltage C terminal for this test.
Bad transformer. Replace.
8) Check for 120 V between L1 and Neutral feeding the control board from the (closed) furnace door switch.
Bad control board. Check for loose or corroded connections before condemning.
9) Check for 120 V at the door switch.
Door switch is open or faulty. Close or replace.
Recheck that the breaker is not tripped and the external furnace switch is ON. If in a basement, check the top of the stairs for a possible second remote furnace switch that may be OFF.