ServiceNomad RV · Knowledge
Suburban Furnace Lockout
When a Suburban furnace enters lockout mode, it stops all ignition attempts and requires manual intervention to reset. Lockout is a safety feature designed to prevent repeated gas releases without successful ignition—a situation that could lead to gas buildup and explosion risk. Understanding what triggers lockout helps you troubleshoot the underlying issue rather than just resetting repeatedly, which can mask a dangerous problem.
What Causes Lockout Mode
Suburban furnaces typically allow three ignition attempts before entering lockout. If the system tries to light three times and fails each time (no flame detected by the flame sensor), the control board assumes a persistent problem exists and shuts down completely. The furnace will not attempt to relight until you manually reset it by turning off the thermostat, waiting 30 seconds, and turning it back on. Lockout prevents the furnace from pumping raw propane into the combustion chamber repeatedly. Each failed ignition attempt releases a small amount of unburned gas—after three attempts, the risk of accumulated gas igniting explosively becomes unacceptable.
Flame Sensor Issues Leading to Lockout
The most common cause of lockout is a dirty or failed flame sensor. The furnace may actually be igniting and burning properly, but if the flame sensor is coated with soot, corrosion, or carbon deposits, it cannot detect the flame. The control board sees 'no flame' and shuts down the gas valve within seconds. After three such cycles, lockout occurs. A technician will remove and clean the flame sensor with fine abrasive material (steel wool or emery cloth) to restore its ability to conduct the microamperage signal back to the control board. In some cases, the sensor wire has broken or developed high resistance, requiring replacement.
Weak Ignition or Intermittent Spark
If the ignition system is marginal—producing weak or intermittent spark, or an HSI that doesn't glow hot enough—the furnace may sometimes ignite and sometimes fail. After enough failures, lockout is triggered. This is often voltage-related: if your house batteries are below 12.0V, the control board and igniter may not receive sufficient power to operate consistently. Charge your batteries and verify you have at least 12.3V at the furnace before troubleshooting further. A technician will also test the ignition module or HSI circuit under load to ensure proper voltage and current draw.
Airflow Restriction and Combustion Issues
If the combustion air intake or exhaust vent is blocked (by insect nests, debris, snow, or ice), the furnace cannot establish proper draft. Even if ignition occurs, the flame will be lazy, yellow, or will blow out immediately due to improper air/fuel mixture. After three such attempts, lockout occurs. A technician will inspect the exterior furnace vents and combustion chamber for obstructions, clean as needed, and verify proper airflow with a pressure or draft gauge. They'll also check that the blower wheel is clean and spinning freely—restricted blower airflow can cause similar symptoms.
⚠️ Safety Notes
- Never repeatedly reset a furnace in lockout without addressing the underlying cause—you risk gas accumulation and explosion.
- If you smell propane after a lockout event, immediately shut off the propane supply, ventilate the RV, and call for help.
💡 Prevention Tips
- Clean the flame sensor and inspect furnace vents at the start of each camping season.
- Ensure your house batteries are fully charged before cold weather camping to prevent voltage-related lockouts.
Can't fix it yourself?
Don't let a mechanical issue ruin your trip. Connect with vetted mobile RV technicians who can come to your location and get you back on the road.