506468-01
Page 19 of 24
Issue 1946
System Diagnostic Module
4HP16LT units contain a diagnostic module for
troubleshooting heat pump system failures. By monitoring
and analyzing data from the compressor and thermostat
demand, the module can accurately detect the cause of
electrical and system related failure without any sensors.
If a system problem occurs, a flashing LED indicator
communicates the failure code.
LED Description
POWER LED (Green) indicates voltage is present at the
power connection of the module.
ALERT LED (Yellow) communicates an abnormal system
condition through a unique flash code. The ALERT LED
will flash a number of times consecutively, pause, and then
repeat the process. The number of consecutive flashes
correlates to a particular abnormal condition.
TRIP LED (Red) indicates there is a demand signal
from the thermostat but no current to the compressor is
detected by the module. The TRIP LED typically indicates
the compressor protector is open or may indicate missing
supply power to the compressor.
Interpreting the Diagnostic LEDs
When an abnormal system condition occurs, the
diagnostic module displays the appropriate ALERT and/or
TRIP LED. The yellow ALERT LED will flash a number of
times consecutively, pause, and then repeat the process.
To identify a flash code number, count the number of
consecutive flashes. Refer to Table 7 for information on
the flash codes.
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 24VAC 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
control board without experiencing nuisance alerts.
Thermostat Wiring
The diagnostic module requires a thermostat demand
signal to operate properly. See Figure 6 for connections.
The thermostat signal input is 24VAC and should not be
less than 0.5VAC.
Mis-wired Module Codes
Depending on the system configuration, some ALERT
flash codes may not be active. The presence of safety
switches affects how the system alerts are displayed by
the module. Mis-wiring the diagnostic module will cause
false LED codes. Table 8 describes LED operation when
the module is mis-wired and what troubleshooting action is
required to correct the problem.
LED Status
Fault Description
Troubleshooting Information
POWER
(Green)
Module has power
Supply voltage is present at module terminals
TRIP
(Red)
Thermostat demand signal Y1 is
present, but the compressor is not
running
1. Compressor protector is open
• Check for high head pressure
• Check compressor supply voltage
2. Outdoor unit power disconnect is open
3. Compressor circuit breaker or fuse(s) is open
4. Broken wire or connector is not making contact
5. Low pressure switch open if present in system
6. Compressor contactor has failed open
ALERT
(Yellow)
Flash Code 1
Long Run Time
Compressor is running extremely long
run cycles
(Not applicable on heat pump models)
--
ALERT
(Yellow)
Flash Code 2
System Pressure Trip
Discharge or suction pressure out of
limits or compressor overloaded
1. High head pressure
• Check high pressure switch if present in system
• Check if system is overcharged with refrigerant
• Check for non-condensable in system
2. Condenser coil poor air circulation (dirty, blocked, damaged)
3. Condenser fan is not running
• Check fan capacitor
• Check fan wiring and connectors
• Check fan motor for failure or blockage
4. Return air duct has substantial leakage
5. If low pressure switch present in system, check Flash Code 1 information
ALERT
(Yellow)
Flash Code 3
Short Cycling
Compressor is running only briefly
1. Thermostat demand signal is intermittent
2. Time delay relay or control board defective
3. If high pressure switch present, go to Flash Code 2 information
4. If low pressure switch present, go to Flash Code 1 information
Table 7. Diagnostic Module Codes
