Lennox International Inc. 4HP18LT - Manual (Page 21)

506470-01 Issue 1007
Every time the module powers up, the last ALERT LED flash
code that occurred prior to shutdown is displayed for
60 seconds. The module will continue to display the previous
flash code until the condition returns to normal or 21VAC is
removed from the module. TRIP and ALERT LEDs flashing
at the same time means control circuit voltage is too low for
operation. 24VAC Power Wiring
The diagnostic module requires a constant nominal 24VAC
power supply. The wiring to the module’s R and C terminals
must be directly from the indoor unit or thermostat. The
module cannot be powered by R and C terminals on the
defrost board without experiencing nuisance alerts.
Page 21
Flash Codes (cont.)
Table 11