High pressure switch closed while the inducer is off. This is a critical fault for gas heat. Cooling can still run. The code is present during a heat call before pre-purge.
Likely causes
- Faulty high pressure switch
- Pressure switch physically bypassed
- Loose or faulty wiring
- Abnormally high negative pressure on the vent with the inducer not running
Safe checks for 55
- Do not jumper or bypass the pressure switch. The manual lists a field bypass as a cause of 55, and says to remove it
- Technicianreplace the high pressure switch if it is faulty, remove any bypass and correct the reason it was added, repair the wiring, and check venting and terminations against the installation instructions
Capacitors can hold a charge after the power is off. Safety terms.
When to call a pro
Call a licensed HVAC technician. A switch that stays closed with the inducer off is a safety-circuit fault. Code 57 is the high pressure switch open with the inducer on high speed.
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Parts & tools for this fix
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Furnace pressure switch: guide
In this code: Faulty high pressure switch
This one is for a technician to test or replace, so we only explain it and do not link to buy it.
Furnace inducer (draft) motor: guide
In this code: Abnormally high negative pressure on the vent with the inducer not running
This one is for a technician to test or replace, so we only explain it and do not link to buy it.