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Error code reference
Lennox Heat Pump Error Code Alternating FAST Flash (Green/Red)
Coil sensor problem: sensor detected open or shorted. Board will not perform demand or time/temperature defrost. System will still heat or cool.
SourceLennox HPXA16 Series Heat Pump - Service Manualp. 13
Diagnostic procedure
The full Alternating FAST Flash (Green/Red) procedure
Step 1 is visible so you can confirm you are on the right page. The remaining steps, the ones that tell you whether to actually replace a part, unlock with any pass.
MyPros+ · DiagnosticsStep 1 preview
1
First diagnostic step
Check coil sensor resistance per Table 4. Replace sensor harness if shorted or open.
SourceLennox HPXA16 Series Heat Pump - Service Manualp. 13
More steps, wiring diagram references, and verified part numbers
Includes expected resistance and voltage values, connector pinouts, and supersession notes for replacement parts.
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Where this code is documented
1 manual in the MyPros+ library documents this code for Lennox heat pump.
Models covered by the manuals that document this code
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