Error code reference

Lennox Heat Pump Error Code Alternating FAST Flash

Coil sensor problem

Coil sensor problem — sensor detected open, shorted, or out of temperature range; control will not perform demand or time/temperature defrost operation (system will still heat or cool).

First step from the service manual

Check coil sensor resistance at pins 5 & 6 (Brown wires); replace sensor harness if open or shorted.

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
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Alternating FAST Flash on my Lennox Heat Pump

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Coil sensor problem — sensor detected open

From Service Manual 641587, p.39:

1.Check coil sensor resistance at pins 5 & 6 (Brown wires); replace sensor harness if open or shorted.

2.Measure control board supply voltage at the test points. Expected: 12 VDC and 5 VDC; a drop below 4.75 VDC on the logic rail points to a failed onboard regulator.

3.Inspect all connectors on the main control for corrosion, thermal stress, or backed-out pins. Reseat each connector with the power disconnected.

Part WPW10804018

Electronic control board

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First step from the service manual

Get the full diagnostic procedure for Alternating FAST Flash.

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