How to Diagnose Ignition Failure in Nortek Gas Furnaces: A Step‑by‑Step Guide

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Nortek gas furnaces, previously Nordyne furnaces, tend to have ignition failures occur when one or more safety verification steps in the direct ignition operating sequence is not working properly during initial startup. When you need to know how to diagnose ignition failure in Nortek gas furnaces, use our guide to ensure you follow the correct steps.

This guide covers steps for MG1, MG, MM, MS, MB, and M7 Series models.

Diagnostic Procedure for Nortek Furnace Ignition Failure

Step 1: Confirm Thermostat Heat Call

Before the ignition process starts, be sure to check the following:

24V control voltage present at the W terminal
Transformer secondary voltage present
No open low‑voltage fuse on control board

Step 2: Check Inducer Motor Operation

If the inducer motor doesn’t have negative pressure in the combustion chamber prior to ignition, the unit won’t start properly. In addition, improper venting and airflow through the burners may cause certain safety switches to open and interrupt the sequence. Some causes of an inducer startup failure include:

Open limit switch
Failed inducer motor
Capacitor failure
Board relay malfunction

Step 3: Verify Pressure Switch Closure

When the startup begins, the airflow through the pressure switch tubing creates a strong vacuum that will cause the switch to close, continuing the ignition process. If that pressure switch doesn’t close, try checking these areas:

Cracked vacuum hose
Intake or exhaust blockage
Condensate drainage restriction
Inducer housing debris
Faulty pressure switch

Step 4: Inspect the Hot Surface Ignitor (HSI)

If the ignitor isn’t energizing, you’ll want to check for:

Resistance outside manufacturer specification
Fractured ignitor element
Open circuit
Improper grounding

Step 5: Confirm Gas Valve Operation

If you notice that the ignitor is energizing but the burners aren’t igniting:

Verify 24V present at gas valve terminals
Listen for valve actuation
Measure manifold gas pressure

Common failure causes include:

Low supply gas pressure
Blocked burner orifice
Valve coil malfunction

Step 6: Diagnose Flame Sensor Operation

Once the burner ignites, the flame sensor monitors the burner operation to detect a flame. If a flame isn’t detected, the control board shuts off voltage to the gas valve to prevent the startup process from continuing. If this happens, inspect for:

Contaminated sensor rod
Improper furnace grounding
Broken sensor lead
Weak microamp signal

Step 7: Evaluate Limit and Rollout Switches

If there’s excessive heat or improper venting, the ignition startup will also fail. The flame rollout switches monitor the burner chamber temperature and may interrupt the sequence if there isn’t proper venting or if the airflow is restricted. Try checking for:

Heat exchanger airflow restriction
Vent blockage
Burner contamination
Failed safety switch continuity

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