Heil furnace 9 flashes: High pressure switch open when it should be closed

Nine flashes means the high stage pressure switch did not close when the furnace called for high fire. Low fire usually still works, so the symptom is a furnace that heats but never reaches full output.

Applies to: International Comfort Products *9MPT and *9MPV two-stage gas furnaces using the Honeywell SV9541Q SmartValve, sold as Heil, Tempstar and Comfortmaker. Single-stage furnaces have no high pressure switch.

What it means

When a two-stage furnace steps up to high fire it moves more combustion air, and a separate pressure switch must close to prove that draft. This code means it did not. The practical consequence is distinctive and easy to misread: the furnace is not dead. It runs on low fire, which in mild weather may be perfectly 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 continuously without reaching setpoint is the usual way people meet this code — and by then the underlying restriction has often been developing for weeks.

Most common causes

• The high stage pressure switch has failed and no longer closes.
• The sensing hose to the high stage switch disconnected, kinked or blocked.
• The inducer not reaching high stage speed, so the switch never sees its rated pressure.
• A partly restricted vent or intake — adequate for low fire, insufficient 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 firing rate.
• The two pressure switches swapped during a previous repair.

How to diagnose it

1. Note whether the house heats slowly rather than not at all. That pattern fits this fault and rules out several others.
2. Check the outside vent termination — a partial restriction is precisely what produces a high-stage-only failure.
3. Check the condensate drain for slow drainage rather than complete blockage.
4. Note whether the fault followed a service visit, since swapped switches are a genuine and easily missed cause.
5. Confirm the furnace is two-stage; a single-stage model showing this means the code has been misread.

How to fix it

1. Clear the vent termination and condensate drain if either is restricted. Both are within homeowner reach and both cause this.
2. The pressure switches, hoses and inducer are technician work.
3. Have the technician confirm the switches are fitted the right way round if the furnace was recently serviced.
4. Do not simply live with low fire and ignore the code. The partial restriction usually worsens, and eventually it affects low fire too.

Notes

A furnace stuck on low fire is easy to tolerate in mild weather and impossible in a cold snap. Acting when the code appears rather than when the house stops keeping up is typically several weeks and one weather change cheaper.

Related

• 8 flashes — High pressure switch closed when it should be open
• 3 flashes — Low pressure switch still open after the inducer started

All codes for this manufacturer

Heil, Tempstar and Comfortmaker furnace 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.

Sources

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