45
I-SCE (09-18) PN207697R9
If the main gas valve fails to open with a normal full-size pilot flame established, check the following:
a. If voltage between black and brown leads on main gas valve is 20–32 VAC and there is no main gas flow with
built-in manual valve in FULL OPEN position, main valve is defective.
b. If there is no voltage between black and brown leads on main gas valve, check for disconnected or shorted
flame sensor lead or flame sensor probe.
If the above conditions are normal and main gas flow is still off, the ignition controller is probably defective. Do not
attempt to service the ignition controller as it does not contain any replaceable components.
CLEANING HEAT EXCHANGER
• Remove the burner rack assembly in accordance with Burner Rack Maintenance to allow access to the inside
of the heat exchanger tubes.
• Clean the heat exchanger tubes using a 1/2-inch diameter furnace brush.
• A mirror and flashlight are helpful for examining the narrow section of each tube. Remove any accumulated dust
and soot.
VENTER MOTOR MAINTENANCE
Power venter motors are permanently lubricated. No oiling is required.
AIR CIRCULATION SYSTEM MAINTENANCE
Check blower pulley and motor pulley to ensure that they are secured to the shaft. Check belt condition and belt
tension (refer to Belts, Blowers, and Drives).
LIMIT CONTROL CHECK
With the heater on, completely block off distribution air. The limit control should open within a few minutes to shut
off the gas supply to the main burners.
TROUBLESHOOTING
Refer to Table 21 to troubleshoot the unit.
NOTE: If the furnace is equipped with electronic modulation option AG39 or AG40, see Figure 29
for additional troubleshooting guidelines.
Table 21. Troubleshooting Table
SYMPTOM PROBABLE CAUSE REMEDY
Venter
motor will
not start
1. No power to furnace Turn on power and check supply fuses or circuit breaker
2. No 24-volt power to venter relay Turn up thermostat and check control transformer output
Check for loose or improper wire connections
3. Defective venter relay Replace defective part
4. Defective motor or capacitor Replace defective part
Pilot will
not light
with venter
operating
1. Manual valve not open Open manual valve
2. Air in gas line Bleed gas line
3. Dirt in pilot orifice Remove pilot orifice and clean with compressed air or solvent
4. Gas pressure too high or too low Adjust gas supply pressure (refer to Gas Piping and Pressures)
5. Kinked pilot tubing Replace tubing
6. Pilot valve does not open If 24-volt power is available at valve, replace valve
7. No spark
a. Loose wire connections Ensure that all wires connections are solid
b. Transformer failure Ensure that 24-volt power is available
c. Incorrect spark gap Maintain spark gap at 0.100 inch
d. Spark cable shorted to ground Replace worn or grounded spark cable
e. Spark electrode shorted to ground Replace pilot if ceramic spark electrode is cracked or grounded
f. Drafts affecting pilot Ensure that all panels are in place and are tightly secured to prevent drafts at pilot
g. Ignition control not grounded Ensure that ignition control is grounded to furnace chassis
h. Faulty ignition controller If 24-volt power is available to ignition controller and all other causes have been
eliminated, replace ignition control
8. Optional lockout device interrupting
control circuit due to above causes
Reset lockout by interrupting control at thermostat
9. Faulty combustion air proving switch Replace combustion air proving switch
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