Troubleshooting Flame Sense Issues on Miller 80% Mobile Home Furnaces

Applicable Models: M1GE, M1MB, MMHB, MG2E, MMHA

If you're dealing with flame dropout issues on a Miller 80% mobile home furnace, follow this systematic troubleshooting approach to quickly identify and resolve the problem.

Step 1: Confirm Flame Sense is the Issue

These control boards typically feature a yellow LED indicator for flame sense. Understanding what the LED is telling you is critical:

Solid Yellow: Strong flame sense signal
Blinking Yellow: Weak flame sense signal
No Light: No flame sense detected

If you have solid yellow flame sense but the furnace is still shutting down, your issue likely lies with a safety control (limit switch or pressure switch), not flame sensing. Look elsewhere in the system.

If you're seeing weak or no flame sense, proceed with the following diagnostic steps in order.

Step 2: Inspect the Vent System

Check the inner pipe of the roof jack (vent system) for holes or deterioration.

This is often overlooked but critical. Holes in the vent pipe allow combustion gases to recirculate back into the burner chamber. This creates a flame with insufficient oxygen—a non-ionizing flame that the sensor cannot detect properly.

What to look for:

Rust holes in the inner vent pipe
Separated joints
Physical damage to vent piping

If you find holes or damage, repair or replace the affected section before proceeding.

Step 3: Examine the Burner Orifice

As strange as it sounds, spiders love to build nests in burner orifices. This is a common issue in mobile home furnaces, especially units that have been sitting idle.

Inspection process:

Remove and visually inspect each orifice
Look for webs, nests, or blockages
Clean or replace as needed

A partially blocked orifice will restrict gas flow and create a weak, unstable flame that won't generate sufficient ionization current.

Step 4: Check the Burner Assembly

Inspect the burner itself for debris, insect nests, or corrosion. Any obstruction in the burner will disrupt flame quality and affect flame sense.

What to check:

Burner ports (clean and unobstructed?)
Burner box area (clear of debris?)
Evidence of rodent or insect activity

A dirty burner produces a poor flame pattern that may not properly envelope the flame sensor, resulting in weak or intermittent flame sense.

Step 5: Test/Replace the Ignitor/Flame Sensor

Once you've verified the vent system, orifice, and burner are all clean and functioning properly, the issue may be a failed ignitor/flame sensor assembly.

Before replacement:

Visually inspect the flame sensor rod for cracks or heavy carbon buildup
Check for proper flame sensor positioning (should be in the flame path)
Verify wiring connections are clean and tight

If everything checks out but you still have weak flame sense, proceed with replacement of the ignitor/flame sensor assembly.

The Whole-System Approach

Remember: effective troubleshooting means keeping the entire system in mind. Flame sense issues are often symptoms of problems elsewhere in the combustion process—poor venting, restricted gas flow, or contaminated combustion air.

Work methodically through these steps rather than jumping to parts replacement. You'll diagnose faster, fix it right the first time, and avoid unnecessary callbacks.

Questions or stuck on a tricky Miller furnace issue? Contact us for technical support.