The Honeywell Q3400A1024 is an igniter flame rod, in other words a pilot light, for a home heater. The main issue with any pilot light is a malfunctioning ignition. In the case of the Q3400 ignition problems are almost always linked to the SmartValve control. A SmartValve control provides gas and the electricity to light it to the Q3400. In order to troubleshoot the Q3400 it's necessary to troubleshoot the SmartValve. Troubleshooting these parts is tricky and often requires replacing problem parts.
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Remove the front cover of the heater to access the SmartValve and Q3400. Set the thermostat several degrees higher than room temperature to initiate the system.
Check the voltage at the control harness by setting the digital multimeter to 120V, attaching the red terminal to the control harness input on the SmartValve and the black terminal to the harness input on the thermostat.
Examine the results, if the voltage reads lower than 24V than the problem is most likely with the wiring and thermostat. Thoroughly examine all the wiring for any obstructions, bends and damage. Replace damaged wires.
Examine the Q3400 burner if the system is receiving 24V. Look for any visible damage such as cracks, bends and general disfigurement. Replace the burner if any visible damage is observed.
Check the voltage of HSI element output and the SmartValve control if the burner appears in working order and the igniter gets hot and glows red. Attach the red terminal of the digital multimeter to the HSI element terminal found right next to the SmartValve and attach to black terminal to the control harness input of the SmartValve.
Replace the SmartValve if the voltage reads less than 24V.