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Status LED Used to Troubleshoot
The LED can be used to check the appliance status. A
description of the LED signals follows.
IMPORTANT:
The LED shows system status for the current call
for heat. When the call for heat is interrupted
(thermostat satisfied or system power switched
off), the LED goes off and the status information
is lost.
• Pulsing heartbeat (on, pulsing) is the normal operating
mode during a call for heat (24V terminal is powered).
The system can be in any of the normal operating
modes including warmup, combustion air proving,
ignition trial, normal run, or between trial delay.
• One flash indicates the S8921D is in an air proving
switch safe start failure mode. This means that the air
proving switch (normally open) is being detected as a
closed circuit. The control is not locked out. Reset the
thermostat and retry. Replace the air proving switch if
indication continues.
• Two flashes indicate the S8921D is in lockout due to
insufficient combustion air flow. Check combustion air
flow. Reset the thermostat and retry. Rectify combus-
tion air flow if indication continues.
• Three flashes indicate a lockout due to ignition failure.
The most probable cause is the main burner failing to
light or failure to detect the flame. Run the appliance
through a call for heat and if the burner does not light,
check the:
—gas supply
—input voltage
—hot surface igniter
—gas control
—wiring
If the main burner lights, but goes out at the end of
ignition trial, check the:
—flame rod for weak flame rectification signal. The
most probable causes are:
—contaminated or mislocated igniter/sensor or flame
rod
—poor flame sense leadwire insulation or connections
—poor gas pressure
—burner ground connections
—wiring
• Four flashes indicate a lockout due to an internal error.
The most probable cause is a S8921D logic failure.
Start a new call for heat and if the four-flash signal
returns, replace the S8921D.
• On steadily indicates a control defect. Start a new call
for heat. If LED comes on steadily (no pulsing heart-
beat) replace the S8921D.
• Off indicates control defect or no power. Check system
power and wiring. If there is 24 Vac between the
S8921D 24V and 24V (GND) terminals, but LED is off,
replace the S8921D.
S8921D HOT SURFACE IGNITION MODULE
Ignition System Checks
Step 1: Check wiring harness.
Make sure:
1. Connections to the ignition module, air proving
switch, and gas control are clean and tight.
2. Hot surface ignition cable provides good electrical
continuity.
Step 2: Check ignition system grounding.
Nuisance shutdowns are often caused by a poor
or erratic ground.
A common ground is required for the module, flame
sensor, and main burner.
1. Check for good metal-to-metal contact between the
igniter bracket and the main burner.
2. Check the ground path from the GND (BURNER)
terminal on the module to the main burner. Make
sure connections are clean and tight. If the wire is
damaged or deteriorated, replace it by following the
appliance manufacturer instructions. A temporary
leadwire connection between the GND (BURNER)
terminal and the main burner can help confirm a
ground path problem.
3. Check temperature at the igniter ceramic or flame
sensor insulator. Excessive temperature will permit
leakage to ground. Contact appliance manufacturer
if temperature exceeds rating of igniter or sensor.
4. If flame sensor or bracket is bent out of position,
restore to correct position.
5. Replace igniter or flame sensor with identical unit if
insulator is cracked.
Step 3: Check flame sensing circuit.
1. Make sure burner flame is capable of providing a
good rectification signal. See Fig. 3.
2. Make sure about 3/4 to 1 in. of the flame sensor is
continuously immersed in the flame for best flame
signal. See Fig. 3.
3. Check for excessive (over 1000°F [538°C]) tempera-
ture at the ceramic insulator on the flame sensor.
Excessive temperature can cause a short to ground.
4. Check for cracked igniter or sensor ceramic
insulator, which can cause short to ground, and
replace unit if necessary.
• Make sure electrical connections are clean and
tight. Replace damaged wire with moisture-
resistant No. 18 wire rated for continuous duty up
to 221°F (105°C).
5. If the igniter is other than a Norton 271, make sure it
meets the following specifications:
• Igniter must reach 1832°F (1000°C) within 17
seconds with 102 Vac applied.
• Igniter must maintain at least 500M ohm insulation
resistance between the igniter leadwires and the
igniter mounting bracket.
• Igniter must not develop an insulating layer on its
surface (over time) that would prevent flame
sensing.
• Igniter surface area immersed in flame must not
exceed one-fourth of the grounded area immersed
in flame. This would prevent flame sensing.
• Igniter current draw at 132 Vac must not exceed 5A.
9
69-0924—2
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