Error code reference
Lennox Heat Pump Error Code Alternating FAST Flash (Green/Red)
Coil sensor problem
Coil sensor problem — sensor detected open or shorted. Board will not perform demand or time/temperature defrost. System will still heat or cool.
First step from the service manual
Check coil sensor resistance per Table 4. Replace sensor harness if shorted or open.
The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.
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Get the full diagnostic procedure for Alternating FAST Flash (Green/Red).
- 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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