Goodman furnace 7 flashes: Flame signal is too weak
Seven flashes means the furnace is lighting and running, but the control can barely detect the flame. It is a warning ahead of a failure — the furnace usually still heats, and soon it will not.
Applies to: Goodman GMS, GMH, GMV8, GMV9 and Amana AMH, AMV9 series gas furnaces with a single red diagnostic LED. On Goodman GMSS9 and Amana AMSS9 the same flash count is described as a low flame sense microamp signal.
What it means
Flame detection works by rectification: the control puts a small voltage on the flame sensor and reads the tiny current that flows through the flame itself. The current is measured in microamps, and the control has a minimum it will accept. This code means the signal is above zero but low enough that the control is flagging it. That makes it unusual and genuinely useful, because it is a predictive fault rather than a failure — the furnace is still working. Left alone, the signal continues to fall until it drops below the threshold, at which point the furnace stops proving flame, shuts off the gas, and eventually locks out with one flash instead.
Most common causes
- The flame sensor is coated with oxide or carbon, insulating it from the flame. This is the most common cause by a wide margin.
- The sensor is bent or has shifted so that it is only partly in the flame.
- Poor grounding at the furnace, since the rectification circuit needs a solid ground path to work.
- A weak or lazy burner flame caused by low gas pressure.
- Restricted combustion air, producing a flame that does not carry current well.
- A cracked ceramic insulator on the sensor, leaking the signal to the cabinet.
- Corroded or loose wiring between the sensor and the control board.
How to diagnose it
- Note that the furnace is probably still heating. This code often appears while everything seems fine, which is exactly when it is most worth acting on.
- Check whether the furnace has begun short-cycling or occasionally failing to stay lit. That is the signal falling further.
- Check the age of the last service. A sensor that has not been cleaned in several seasons is the usual explanation.
- If the fault appeared alongside other electrical oddities, mention that to the technician — poor grounding causes this and several other faults at once.
How to fix it
- Book a service call before the furnace fails rather than after. This is the rare code that gives you warning, and the repair is usually inexpensive.
- Cleaning a flame sensor is technician work. It sits directly in the burner flame, in the circuit that decides whether gas keeps flowing, and reinstalling it in the wrong position produces the failure it was meant to fix.
- Have grounding checked at the same visit if the sensor has been cleaned recently and the code has returned.
- If gas pressure is low, that needs correcting at the valve or the supply — also licensed work.
Notes
A flame sensor is a plain steel rod with a ceramic insulator and no moving parts, and cleaning it takes minutes. It is worth knowing that this is a small job, because the code's other outcome — being left until the furnace locks out on the coldest night — is the expensive version of the same repair.
Related
- 1 flash — Lockout after too many failed ignition attempts
- 8 flashes — Igniter circuit fault or poor grounding
All codes for this manufacturer
Goodman and Amana furnace fault codes — every code, with what differs between models.
Not sure this is your brand?
The same count means something else on other furnaces. What 7 flashes means on every brand puts them side by side.