Lennox SLP98UHV Error Code C: Cooling Stage Display
What Does Code C Mean?
The letter C in the SureLight control's diagnostic menu reports the cooling stage configuration. It is reached by cycling the push button on the control board, and the LED shows the cooling stage number (1 or 2) followed by the CFM setting for that stage before repeating.
This is informational, not a fault. Although the SLP98UHV is a gas furnace, its control board also runs the indoor blower for your air conditioner or heat pump, so it stores the cooling airflow settings even though the actual cooling happens at the outdoor unit. Stage 1 is the lower-capacity mode used for moderate cooling, and Stage 2 is full capacity for hot days, each with its own blower speed so airflow matches the cooling output.
Nothing needs to be done. The display cycles on to other diagnostic items or returns to idle by itself. Technicians use the C readout to verify the cooling configuration matches the outdoor unit installed with the furnace.
What You'll Notice
- The LED shows the letter C followed by a stage number (1 or 2) and then a CFM value, repeating the sequence.
- System operation does not change while this cycles; the furnace or air handler keeps running normally.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Diagnostic push button pressed to view cooling stage settings | Most common | — |
How This Is Diagnosed
Verify this is only a readout by confirming it appears while paging through the diagnostic button menu. Release the button and let the five-second-per-item menu advance past the C entry; when the display returns to the idle decimal or the active operating indication, the cooling-stage readout has simply timed out and no fault exists.
- You believe the cooling stages or their airflow are not configured to match your outdoor unit
- Cooling comfort is poor and you want the stage configuration verified
Frequently Asked Questions
Is C on my furnace an error code?
No. C is a diagnostic readout of the cooling stage (1 or 2) and its airflow. It is shown deliberately through the diagnostic button, not because of a fault.
Why does my gas furnace show a cooling setting?
The SLP98UHV's control board runs the indoor blower for your air conditioner or heat pump, so it stores and can display the cooling stage settings even though the cooling itself happens at the outdoor unit.
How do I get rid of the C display?
Stop cycling the diagnostic menu and it will time out on its own, returning to the normal display. There is nothing to reset.
✓ Verified against manufacturer service manual — March 2026