Goodman furnace EF: Auxiliary switch or input is open
EF means a device wired into the furnace's auxiliary safety input has opened the circuit. What that device is depends entirely on how the furnace was installed.
Applies to: Goodman GMVC96, GMVM97, GCVM97 and Amana AMVM97 series gas furnaces with a two-character alphanumeric display. This code does not appear in the Goodman GCVC96 or Amana AMVC96 tables, so a furnace of either of those models does not produce it. Goodman describes it as an auxiliary switch open on GMVC96 and as an auxiliary input open on the modulating boards.
What it means
The control provides an auxiliary input so that an additional safety device can stop the furnace. What gets wired into it varies by installation: most often a condensate overflow switch, sometimes an auxiliary limit, occasionally an external interlock. This makes EF unusual among fault codes — it does not identify a specific component, because the component is whatever the installer connected. The most common single cause by a wide margin is a condensate overflow switch, which trips when the drain system backs up and water rises where it should not. That is a real fault worth catching: an unchecked condensate overflow will eventually find the electronics or the floor.
Most common causes
- A condensate overflow switch tripped because the drain line or trap is blocked.
- A full or overflowing condensate pump, or a pump that has failed.
- A frozen condensate line on an installation where it runs through unheated space.
- An auxiliary limit switch that has opened.
- A wiring fault or loose connection in the auxiliary circuit.
- An external interlock device tied into the input, depending on the installation.
How to diagnose it
- Look for water first. A wet furnace base, a full condensate pump reservoir, or water around the drain is the usual answer.
- Check whether the condensate drain line is blocked or, in cold weather, frozen where it passes through an unheated area.
- Check whether a condensate pump is running when it should. A pump with a failed float or a blocked outlet backs up quickly.
- Establish what is actually wired to the auxiliary input if the furnace has an installation record. On an unfamiliar installation a technician will trace it.
How to fix it
- Clearing a blocked condensate drain or emptying a backed-up pump reservoir is reasonable homeowner work and resolves most instances of this code.
- A frozen condensate line needs thawing and then insulating or rerouting, or it will freeze again on the next cold night.
- A failed condensate pump is replaced — straightforward work, though it is plumbing plus electrical.
- If no water is present, the auxiliary circuit needs tracing by a technician, since what is connected varies by installation.
- Do not bypass the switch to restore heat. It is usually protecting against water reaching the furnace's electrical components.
Notes
Because the auxiliary input has no fixed meaning, this is one code where knowing your own installation matters more than knowing the code. If a technician has been to the furnace before, asking what is wired to the auxiliary input is worth doing while everything is working.
Related
All codes for this manufacturer
Goodman and Amana furnace fault codes — every code, with what differs between models.