Lennox E125: control failed self-check (hardware fault)

Furnace control display or LED

E125

“Control failed self-check, internal error, failed hardware. Will restart if error recovers. Integrated control not communicating. Covers hardware errors (flame sense circuit faults, pin shorts, etc.).”

Printed on 16 model series · 16 documents

SL280*V, SL280*NV, SL297*NV, EL296*V, EL296*E, EL297*V, EL297*E, EL280*E, EL195*NE, EL196*E, EL180*NE, ML180*, ML193*E, ML196*E, ML296*V, ML297*V · 508559-01 p. 28; 508561-01 p. 35; 508585-02 p. 48; 1126-L5 p. 16; 1246-L10 p. 13; 508583-01 p. 51; 508577-02 p. 42; 1901-L1 p. 14; 508575-02 p. 46; 508573-01 p. 45; 508545-01 p. 28; 508547-01 p. 48; 508565-01 p. 42; 508569-01 p. 45; 508521-01 p. 41; 508907-01 p. 45

Technician

What it means

E125 means the integrated furnace control found a fault in its own hardware. The wording covers hardware errors such as flame sense circuit faults and pin shorts, and says the control will restart if the error recovers. The SL297UHNVK table describes it as a hardware problem on the control: the action is to cycle power on the control and to replace the control if the problem prevents service and is persistent. The table calls it a critical alert that clears 300 seconds after the fault recovers 508585-02, p. 48. The SLP99DFVK table gives the same two steps without the clearing time 508589-02, p. 45. The compact ML180UHVK table puts E125 in the group “HARDWARE FAULT (5 MINUTES LOCKOUT)” 508547-01, p. 26.

The power cycle in the action column is part of a table written for the installer. Lennox’s owner’s manuals give no reset step for any code.

Lennox’s thermostat alert 125 is “Control Hardware Problem”, priority Service Soon / Service Urgent. Its description is a hardware problem on a system component control, so it is not limited to the furnace 100017, p. 17. Lennox’s documents do not say that alert 125 and display code E125 report the same event.

Check these first

Lennox’s owner’s manuals have no code table and give no steps for E125. What they give is a general list for a unit that will not operate, to work through before calling a Lennox service technician. In the SLP99DFVK manual the order is:

Check that the electrical disconnect switches are ON 508590-01, p. 5.
Check the room thermostat for proper setting 508590-01, p. 5.
Replace any blown fuses or reset circuit breakers 508590-01, p. 5.
Check that the gas valve is ON 508590-01, p. 5. Its switch is behind the outer access panel; the manual’s gas valve steps set the thermostat to the lowest setting and turn off all electrical power to the unit before that panel comes off, and the switch is moved by hand only, never with tools 508590-01, p. 4.
Check that the air filter is not plugged, which would limit air flow 508590-01, p. 5. The SLP99DFVK manual says its filters are installed external to the unit, should be inspected monthly, and that a filter must be in place when the unit is operating 508590-01, p. 4. Before any service or maintenance, it says to turn the electrical power to the unit OFF at the disconnect switch 508590-01, p. 2.
Check that gas is turned on at the meter 508590-01, p. 5.
Check that the manual main shut-off valve is open 508590-01, p. 5.

Then record the model and serial number from the nameplate and call 508590-01, p. 5. The list is not tied to any code. More on it, and on how other Lennox manuals word it: owner checks before a service call.

The manual’s Maintenance section says the maintenance procedures should only be conducted by a licensed professional service technician, and not to attempt to service the unit in any way 508590-01, p. 5.

When to call a licensed technician

Cycling power on the control and replacing the control if the problem prevents service and persists 508585-02, p. 48; 508589-02, p. 45.
Any work inside the furnace 508590-01, p. 5.
The furnace still will not run after the checklist 508590-01, p. 5.
E125 keeps coming back after it clears. The owner’s manuals give no step for that; these pages send it to the technician.

Notes for different models

Series or document	Code	Wording	Source
E-code tables of the SL280, SL297, EL280, EL296, EL297, EL180, EL195, EL196, ML180, ML193, ML196, ML296 and ML297	E125	“…Integrated control not communicating…” The action ends with the critical alert and the 300-second clearing time.	508585-02, p. 48; 508569-01, p. 45; 508521-01, p. 41
SLP98V, SLP99V, G71MPP	E125	“…Integrated furnace control not communicating…” Action: “Hardware problem on the control board. Cycle power on control. Replace if problem prevents service and is persistent.”	508589-02, p. 45; 506610-01, p. 62; 506509-01, p. 65
ML180UHVK, compact table	E125	“CONTROL HARDWARE (INCLUDE A/D ELECTRONICS AND FLAME TEST) FAILURE”, in the group “HARDWARE FAULT (5 MINUTES LOCKOUT)”	508547-01, p. 26
Lennox thermostat alert table	125	“Control Hardware Problem”, Service Soon / Service Urgent	100017, p. 17

The red LED tables have no E125, and their steady states read differently from table to table:

Table	LED off	LED on	Source
13-flash tables, boards 103217-03 and 107163-01	“No power to control or board fault detected”	On the 107163-01 board: “Board Fault Detected”	507966-04, pp. 40–41
9-flash tables: ML193UH, ML195UH, ML296UHV	“No power to control or control harware fault detected.” (as printed; ML296UHV: “hardware”)	“Normal operation.”	507121-01, p. 39; 507268-03, p. 39; 507869-04, p. 46
ML180DF, 2018	Listed twice: “No power to control or control harware fault detected.” and “Normal operation.”	Not listed	507326-01, p. 21