American Standard furnace error codes

E01 means the integrated furnace control (IFC) has detected an internal failure or an IRQ loss. The fault lies in the control board's own operation.

E2.1 means the furnace made three ignition attempts in one heat call and never sensed flame. The control then enters a one-hour lockout.

E2.2 means flame was established and then lost, and this repeated until ten recycles accumulated in one heat call. The control then enters a one-hour lockout.

E2.3 means the control did not detect 24 VAC at the first-stage gas valve (MVL) when it was required, ten times in one heat call. The manual index states this backwards; the main chart and detailed page agree.

E3.1 means the first-stage pressure switch is closed when the control expects it to be open. The manual notes that the switch itself is usually not the root cause.

E3.2 means the first-stage pressure switch is open when the control expects it to be closed, so the required draft has not been proven.

E04 means one of three safety switches is open: the main thermal limit, a flame rollout switch or the reverse-airflow switch. The code does not say which one tripped.

E05 means the control sensed flame at a time when no flame should be present. It is a safety fault, not a normal operating state.

E6.1 means the control detected reversed line-voltage polarity at the furnace power supply. It is an electrical supply fault, not a burner fault.

E6.2 means the control has detected a poor ground connection. It points to the furnace's ground path, which needs electrical testing.

E6.3 means the igniter relay has failed or the igniter is open. The detailed diagnostic page also covers a relay stuck closed and an igniter circuit that is open or shorted.

E7.1 means 24 V is present at the first-stage gas valve circuit when it should be off. The manual index lists this as E07; that is a document inconsistency, not a confirmed alternate code.

E08 means flame current is low, below the 0.5 microamp DC threshold in the detailed diagnostics. The furnace is still allowed to operate; this is not a lockout.

On the S8X1, E11 means the first-stage gas valve was not energized when it should have been, or the first-stage valve relay is stuck closed. Second-stage conditions in the chart do not apply.

E12 means the 5 A fuse on the control board is open or missing. An open fuse suggests something in the low-voltage circuit drew too much current.

IdL means idle: there is no cooling, heating or fan demand from the thermostat. The status reflects thermostat inputs, so it is normal when nothing is being called.

Ht means the control sees a gas-heat demand from the thermostat. It reflects the 24 VAC input only and does not confirm the burner lit or that heat is reaching the rooms.

CP1 means the thermostat is calling for first-stage cooling or heat-pump operation. It is a normal demand display, not a furnace fault.

CP2 means the thermostat is calling for second-stage cooling or heat-pump operation. On the S8X1 this is a normal demand display, not a furnace fault.

COf means the thermostat is calling for continuous fan. The blower runs without a heating or cooling call, so register air stays near room temperature.

dFt means the outdoor heat pump has a defrost demand and the furnace is in gas-heat mode during that period. It is a normal demand state, not a fault.