Error code reference

Lennox Heat Pump Error Code Simultaneous FAST Flash

Ambient sensor problem

Ambient sensor problem — sensor detected open, shorted, or out of temperature range; control reverts to time/temperature defrost operation (system will still heat or cool).

First step from the service manual

Check ambient sensor resistance at pins 3 & 4 (Black 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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Simultaneous FAST Flash on my Lennox Heat Pump

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Ambient sensor problem — sensor detected open

From Service Manual 561139, p.31:

1.Check ambient sensor resistance at pins 3 & 4 (Black 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 Simultaneous FAST Flash.

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