Error code reference
Trane Furnace Error Code 3 Double
2nd stage pressure switch open
2nd stage pressure switch open. Furnace reverts back to 1st stage heating.
First step from the service manual
Cycle power to furnace and call for 2nd stage heat; verify all wiring to pressure switch is correct per wiring diagram.
The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.
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- Step-by-step tests with expected resistance, voltage, and continuity values
- Wiring diagram references and connector pinouts from the OEM service manual
- Verified part numbers when replacement is required
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