Heil furnace LED off: The control is not powered

A dark LED on the gas valve means the SmartValve has no power. That is a supply problem rather than a valve problem, and the usual causes are the ones a furnace uses to switch itself off deliberately.

Applies to: International Comfort Products *9MPT and *9MPV gas furnaces using the Honeywell SV9541Q SmartValve, sold as Heil, Tempstar and Comfortmaker.

What it means

ICP documents this simply as the control not being powered. Because the LED sits on the gas valve rather than the control board, a dark light means power is not reaching the valve — which can be a break anywhere upstream, including the furnace control board that feeds it. Before assuming a component has failed, it is worth knowing how many ways a furnace removes its own power on purpose: the blower door switch cuts supply whenever the panel is off, the service switch looks exactly like a light switch, and there is usually a fuse on the furnace control board.

Most common causes

The blower compartment door is off or not seated, opening the door switch.
The furnace service switch has been turned off. It looks like an ordinary light switch and is frequently mistaken for one.
The circuit breaker for the furnace has tripped or been switched off.
A blown fuse on the furnace control board, usually caused by a short in the low voltage wiring.
A failed transformer, so there is no 24 volt supply.
A wiring fault between the furnace control and the gas valve.
A failed SmartValve control.

How to diagnose it

Check the service switch near the furnace first. It is the most common answer and takes a second.
Check the breaker panel for a tripped furnace breaker.
Check whether the furnace control board shows any indication. If the board is alive and the valve is dark, the break is between them.
Note whether the thermostat display is blank, which points at the low voltage side.
Note whether the breaker trips again immediately after being reset. That is a fault, not an inconvenience.

How to fix it

Turn the service switch back on, or reset the breaker, if either was off.
Refit the blower door properly. A door that is not seated holds the switch open and nothing will run.
If the breaker trips again immediately, stop and call a technician.
Wiring between the control board and the gas valve, a blown board fuse, a failed transformer or a failed SmartValve are all technician work. A blown fuse in particular means a short somewhere, and replacing it without finding the short blows the next one.
If the furnace or the area around it has been wet, do not power it up. Call a technician.

Notes

The door switch catches nearly everyone at least once. Unlike a control board LED, which is usually readable through a view port, the gas valve LED often requires the panel off to see — which is exactly what removes its power.

All codes for this manufacturer

Heil, Tempstar and Comfortmaker furnace codes — every code, with what differs between models.

This code is not a homeowner repair. Faults in this group involve the gas valve, the burner compartment, or the heat exchanger — the parts whose failure can put combustion products, including carbon monoxide, into the air the house breathes. Do not reset it repeatedly to keep the heat on. Have it looked at by a licensed technician, and make sure there is a working carbon monoxide alarm on every level of the home, including the level the furnace is on.

Sources

International Comfort Products *9MPT and *9MPV installation instructions — Gas Valve Diagnostic Codes (third-party archive of the ICP document)

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.