Goodman furnace 9 flashes: High stage pressure switch stuck open
Nine flashes means the second-stage pressure switch did not close when the furnace tried to run on high fire. Low-stage heating usually still works, so the symptom is a furnace that heats but never gets to full output.
Applies to: Goodman GMV8 and GMV9 and Amana AMV9 two-stage gas furnaces with a single red diagnostic LED. Single-stage models have no high stage switch and do not produce this code.
What it means
A two-stage furnace has two pressure switches, one for each firing rate, because high fire moves more combustion air and needs its own proof of draft. This code means the high-stage switch failed to close when the control called for second-stage heat. The practical consequence is distinctive: the furnace is not dead. It runs on low fire, which on a mild day may be entirely adequate, so the fault often goes unnoticed until the weather turns and the house cannot keep up. A furnace that used to satisfy the thermostat and now runs constantly without reaching setpoint is the usual way people meet this code.
Most common causes
- The high-stage pressure switch has failed and no longer closes.
- The sensing hose to the high-stage switch is disconnected, kinked, or blocked.
- The inducer is not reaching high-stage speed, so the switch never sees its rated pressure.
- A restricted vent or intake that is adequate for low fire and not for high fire — which is exactly why the fault appears only at second stage.
- A partly blocked condensate drain, which restricts more at the higher rate.
- The two pressure switches swapped during a previous repair.
- A wiring fault in the high-stage switch circuit.
How to diagnose it
- Confirm the furnace is actually two-stage. A single-stage model producing nine flashes means you have miscounted.
- Note whether the house heats slowly rather than not at all. That pattern fits this fault and rules out several others.
- Check the vent termination outside, as with any pressure switch fault. A partial restriction is exactly what produces a high-stage-only failure.
- Check the condensate drain for slow drainage rather than complete blockage.
- Note whether the fault appeared after a service visit, since swapped switches are a real and easily missed cause.
How to fix it
- Clear the vent termination and the condensate drain if either is restricted. Both are within homeowner reach and both cause this.
- The pressure switches, hoses, and inducer are technician work, for the same reason as every other pressure switch fault — they prove safe venting.
- Have the technician confirm the two switches are fitted the right way round if the furnace was recently serviced.
- Do not run the furnace indefinitely on low fire and ignore the code. The partial restriction that causes it usually gets worse, and at some point it will affect low fire too.
Notes
A furnace stuck on low fire is easy to live with in mild weather and impossible to live with in a cold snap. This code is worth acting on when it appears rather than when the house stops keeping up, because the two are typically several weeks and one weather change apart.
Related
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 9 flashes means on every brand puts them side by side.