Burner controls, ignition and flame safeguards
Honeywell 7800 Series Burner Control (RM7800 / RM7840) fault codes
What all 8 Honeywell 7800 Series Burner Control (RM7800 / RM7840) fault codes mean, in plain words, with a risk grade for each. These come from the manufacturer's own manual. The fix for each one is in WERCS Diagnose.
Get the fixes · plans from $49/moUpdated July 22, 2026
| Code | What it means | Risk |
|---|---|---|
| *Running ILK* | Running ILK The burner locked out because a required safety device in the running interlock circuit failed to close during prepurge. | RED |
| *High Fire Sw.* | High Fire Sw. The burner locked out because the control could not confirm the damper reached the fully open high fire position for purging. | RED |
| *Airflow Switch* | Airflow Switch The control did not get proof of proper combustion airflow during the prepurge cycle, which causes the burner control to fault. | RED |
| *Flame Detected* | Flame Detected The control saw an unexpected flame signal while the shutter was open during prepurge, which causes the burner control to lock out. | RED |
| *Main Flame Fail* | Main Flame Fail The burner locked out because the control module lost the main flame signal during normal operation. | RED |
| *Preignition ILK* | Preignition ILK The burner locked out because a preignition safety interlock was not in its expected state before starting. | RED |
| *Low Fire Sw. Off* | Low Fire Sw. Off The burner locked out because the control could not confirm the burner reached the low fire position during prepurge. | RED |
| *Pilot Flame Fail* | Pilot Flame Fail The burner locked out because the control module failed to detect a pilot flame during the ignition period. | RED |
Every code comes from the manufacturer's own manual. Subscribers see which manual and page.