Low pilot voltage. The air conditioner or heat pump may appear to be operating normally. The chart's note says the control detects secondary voltage less than 18 VAC.
Likely causes
- Secondary voltage less than 18 VAC
- Transformer overloaded
- Low line voltage
Safe checks for 09
- Write down 09. This is the low-voltage secondary circuit, not line-voltage code 08 or L8
- Techniciancheck the fuse, correct the low secondary voltage, and check the transformer. The chart says the fault clears if secondary voltage rises above 21 VAC
Capacitors can hold a charge after the power is off. Safety terms.
When to call a pro
Call a licensed HVAC technician. The 18 VAC and 21 VAC figures are the PCBHR104 notes for 09. They are meter readings, not a homeowner test.
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Parts & tools for this fix
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Multimeter for appliance and HVAC diagnosis: guide
In this code: Secondary voltage less than 18 VAC
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