Error code reference
Lennox Heat Pump Error Code Simultaneous FAST Flash (Green/Red)
Ambient sensor problem
Ambient sensor problem — sensor detected open or shorted. Board reverts to time/temperature defrost operation; system will still heat or cool.
First step from the service manual
Check ambient 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 Simultaneous 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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