Your troubleshooting situation this month involves an 80% single stage gas furnace that has been in service approximately seven years. When you respond to the customer’s complaint of “no heat,” you find that the thermostat is set correctly, and the fault code signal from this unit’s control system (see the pictorial diagram) is showing seven red flashes.

After disconnecting the power supply, checking all connections you determine that they are OK. Consulting the manufacturer’s service information and determining a “Failed to light burners” condition, you decide to begin your troubleshooting procedure by attaching your voltmeter at connections 2 and 4 of the HSI circuit on the S2 segment of the control. When you restore power and initiate a call for heat, you note the following:

  1. The IDM starts and operates normally.
  2. Your voltmeter reads 0-Volts.
  3. The fault signal repeats.        

You troubleshooting question: What is the next step you need to take to get this furnace back on line?  

Submit your answer here.

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