Error code reference

Trane Furnace Error Code 4 Flash

High temperature limit fault

High temperature limit fault - open high limit (heat exchanger temperature too high, possibly caused by low airflow or fan failure), auxiliary limit (reverse flow), or rollout switch open.

First step from the service manual

Scroll User Interface to view Last 4 Faults to determine HIGH LIMIT, AUX LIMIT, or ROLLOUT OPEN; disconnect power and check filter/blower wheel cleanliness.

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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4 Flash on my Trane Furnace

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High temperature limit fault - open high limit

From Service Manual 751426, p.38:

1.Scroll User Interface to view Last 4 Faults to determine HIGH LIMIT, AUX LIMIT, or ROLLOUT OPEN; disconnect power and check filter/blower wheel cleanliness.

2.Inspect the communication harness between the main control and the UI board for bent pins or unseated connectors at each end.

3.Measure supply voltage to the UI board. Expected: 5 VDC at the interface connector; a low or missing rail points back to the main control supply.

Part W10811169

User interface control board

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First step from the service manual

Get the full diagnostic procedure for 4 Flash.

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