I have an old Lennox natural gas furnace (model #2003/4E-100-1) which is not igniting properly. When the furnace is cold, a call for heat results in the damper opening but no spark activity. Switching power off and back on results in 24 volts at the pilot valve, a short sparking cycle (maybe 3 sparks) but no ignition . Cycling the power again there is a longer spark cycle but still no ignition. Repeating 5 or 6 times finally ignites the pilot (sustained voltage at valve) and the normal start up sequence follows. If I then turn the thermostat down, and wait up to an hour or so before increasing the thermostat, the furnace will ignite normally. Waiting longer will result in the problem I described above.
My guess is that the ignition module is faulty (Robershaw SP735L) but I'm wondering if it could be the gas valve (Robertshaw 7100) instead.
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